Can someone do Upanayan ritual later in life say post 25 years of age?
Yes, but you have to do prayaschitta before that.
It is true that after the age limit, one cannot normally do Upanayana.
For the Brāhmaṇa the Sāvitrī does not lapse till the sixteenth year; for the Kṣatriya till the twenty-second year; and for the Vaiśya till the twenty-fourth year.—(38)
Beyond this, all these three, not having received the sacrament at the proper time, become excluded from Sāvitrī (initiation), and thereby come to be known as ‘Vrātyas’ (apostates), despised by all good men.—(39)
The Brāhmaṇa should not in ant case, even in times of distress, establish spiritual or uterine relationship with these persons, until they have been duly purified.—(40)
However, as verse 40 says, you can do the appropriate prayaschitta and then do the Upanayana.
The Smritis describe the various prayaschittas:
Āpastamba-Gṛhyasūtra, 1.1.33; 1.2.6.—‘With these (apostates) one should avoid association, dining and marriage; in the event of their being desirous of expiating (the omission and the consequent apostacy), they should keep, for twelve years, the observances relating to Vedic studies, and then undergo the Upanayana, and be sprinkled with water with Pāvamānī and other mantras.’
Manu 11.191 - Though twice-born men to whom the Sāvitrī has not been taught according to rule, should be made to perform three Kṛcchra penances and then initiated in due form.—(191)
Viṣṇu (54-26).—‘Those twice-born men by whom the Gāyatrī has not been
repeated, nor the other ceremonies performed, as the law directs, must
be made to perform three Prājāpatya penances and then initiated
according to custom.’
Āpastamba (1.1.28-29).—‘If the proper time for initiation has passed,
he shall observe, for the space of two months, the duties of a
Student, as observed by those who are studying the three Vedas; after
that he may be initiated; and after that he may be instructed.’
Vaśiṣṭha (11.76-78).—‘A man who has missed the Sāvitrī may undergo the
Uddālaka penance. Let him subsist, during two months, on barley-gruel,
during one month on milk, during half a month on curds, during eight
days on clarified butter, during six days on alms given without
asking, and during three days on water; and let him fast for one day
and night. Or, he may go to bathe with the priests at the end of an
Āśvamedha sacrifice. Or, he may perform the Vrātya-stoma.’