As mentioned in another post, Vishnu's human avataras don't seem to age beyond young adulthood.
Are there scriptural verses putting down old age?
Are there scriptural citations denigrating old age?
There are verses criticizing the suffering due to old age, but there are many verses that say old people must be respected since age is a sign of rank and respect.
From the Manusmriti:
2.134 - Among citizens friendship and equality are regarded as ranging within ten years (of age-difference); among artists, it is regarded as ranging within five years; among learned men, it proceeds up to three years; and among blood-relations, it ranges only within a very short period of time.
2.136 - Wealth, Relation, Age, Action and Learning, as the fifth,—these are the grounds of respect; (among them) that which follows is weightier (than that which goes before it).
2.137 - Among the three (higher) castes, he, in whom there are present most of these five, and of high degree, deserves (greater) respect; as also the Śūdra who has reached the tenth stage (of life).
Gautama-Dharmasūtra (10.6).—‘The Śūdra also [should be respected] if he has children and is over 80 years in age.’
Yājñavalkya (1.116).—‘When the said qualities (of Learning, [age], etc.) are present in a very large degree [a person must be respected]. The Śūdra also deserves respect, when he has reached old age.’
2.138 - Way should be made for one in a chariot, for one who is in the tenth stage of life, for one suffering from disease, for one carrying a burden, for a woman, for the person who has just passed out of studentship, for the king and for the bridegroom.
2.155 - Among Brāhmaṇas seniority is by knowledge; among Kṣatriyas by valour; and among Vaiśyas by grains and riches; among Shudras alone it is by age.
Verses that criticize the suffering due to old age:
Manu 12.79 - Separations from relations and loved ones, dwelling with the wicked, acquiring wealth and losing it, making of friends and enemies.—(79)
Manu 12.80 - Inevitable decrepitude, the pangs of sickness, various forms of afflictions and unconquerable death.
From the Chhandogya Upanishad:
And if they should say to him: 'If everything that exists is contained in that city of Brahman, all beings and all desires (whatever can be imagined or desired), then what is left of it, when old age reaches it and scatters it, or when it falls to pieces?'
Then he should say: 'By the old age of the body, that (the ether, or Brahman within it) does not age; by the death of the body, that (the ether, or Brahman within it) is not killed. That (the Brahman) is the true Brahma-city (not the body 1). In it all desires are contained. It is the Self, free from sin, free from old age, from death and grief, from hunger and thirst, which desires nothing but what it ought to desire, and imagines nothing but what it ought to imagine. Now as here on earth people follow as they are commanded, and depend on the object which they are attached to, be it a country or a piece of land.
In addition to Ikshvaku, I just want to add few lines from Charpatpanjarika-stotra by Sankaracharya(Stavakusumanjali, Udbodhan, page 389:
angam galitam palitam mundam, netravihinam jatam tundam/ vriddho yati grihitwa dandam, tadapi na munchati ashabhaandam//
meaning : the limbs got distorted, the head without hair, mouth without teeth, walking with a stick.Such is the condition at old age, but still he is not able to leave the desires.
The verse in fact is not denigrating old age, but making us aware if the ugliness of the presence of desires at old age.
These verses we all have to remember, so that no one can denigrate us at old age.
The aged persons or the Vriddha-s are considered as ruling the middle sphere (the Antariksha)..
Therefore the question of belittling them is out consideration.
They rule the mid-sphere:
Manu Smriti 4.184. Infants, aged, poor and sick men must be considered as rulers of the middle sphere, the eldest brother as equal to one’s father, one’s wife and one’s son as one’s own body,
One should avoid quarrel with them
4.179. With an officiating or a domestic priest, with a teacher, with a maternal uncle, a guest and a dependant, with infants, aged and sick men, with learned men, with his paternal relatives, connexions by marriage and maternal relatives, .... let him avoid quarrel.
They should not be insulted:
4.141. Let him not insult those who have redundant limbs or are deficient in limbs, nor those destitute of knowledge, nor very aged men, nor those who have no beauty or wealth, nor those who are of low birth.
A king should not punish them or treat them harshly:
8.312. A king who desires his own welfare must always forgive litigants, infants, aged and sick men, who inveigh against him.
8.395. Let the king always treat kindly a Srotriya, a sick or distressed man, an infant and an aged or indigent man, a man of high birth, and an honourable man (Arya)
Why would old age be criticized when it is nothing but the natural consequence of birth? If something that is criticized then that is people's inability to see that with every moment passing by they are getting older and closer to death.
Something like this:
Swah kAryamadya kartyavm purvAnhe chAparAnhikam |
Na hi pratikshate mrityuh kritam vAhasya na vA kritam ||
Whatever to be done tomorrow is to be done today; whatever is to be done in the noon should be completed in the morning only. Death does not wait to see what has been done by this person and what he is yet to do.
KulArnava Tantram 1.42
In the Ramayana, jambavan regrets the loss of youthful strength:
त्रिविक्रमे मया तात स शैल वन कानना |
त्रिः सप्त कृत्वः पृथिवी परिक्रान्ता प्रदक्षिणम् || ४-६६-३२
"During the period of Trivikrama incarnation I have circumambulated the earth inclusive of its mountains, forests and thickets for twenty-one times, moving rightward around it... [4-66-32]
तथा च ओषधयो अस्माभिः संचिता देव शासनात् | निर्मथ्यम् अमृतम् याभिः तदा तदानीम् नो महत् बलम् || ४-६६-३३
"Like that, by the order of gods we ourselves have collected those herbal plants with which ambrosia had to be churned from Milky Ocean, and in that way, then there used to be a great energy for us... [4-66-33]
स इदानीम् अहम् वृद्धः परिहीन पराक्रमः |
साम्प्रतम् कालम् अस्माकम् भवान् सर्व गुण अन्वितः || ४-६६-३४
"Such as I am, I am now aged and my valiance is completely lessened... but at the present time, you alone are endowed with all endowments among us... [4-66-34]
Then there is the story of Yayati
One day, Devayani met the three sons of Sharmishtha. She asked the boys the name of their father. They pointed to Yayati. Devayani was shocked. She felt deceived and ran to her father's hermitage. Sukracharya was enraged and cursed Yayati with premature old age. Yayati begged for forgiveness. Sukracharya and Devayani felt sorry for him. Sukracharya then said, "I cannot take back my curse, but if any of your sons is ready to exchange his youth for your old age, you will be young again as long as you wish."
Yayati, now an old man, quickly returned to his kingdom and called for his eldest son. "My dutiful son, take my old age and give me your youth, at least for a while, until I am ready to embrace my old age." The eldest son turned down his father's request and so also the next three older brothers. Then came the youngest, Puru. He agreed and immediately turned old. Yayati rushed out as a young man to enjoy his life. After years spent in vain effort to quench his desires by indulgence, Yayati finally came into senses. He returned to Puru and said, "Dear son, sensual desire is never quenched by indulgence any more than fire is extinguished by pouring oil on it. Take back your youth and rule the kingdom wisely and well."
Old age is considered a curse and the other sons wouldn't exchange their youth for it.