Body can move automatically ? Or there is somebody or now days or may be confidential God is moving ? If all this questions answered is yes GOD is created and moving. Then one last question what we are doing in this miserable world with three types of life childish, youngish and old age with limited life ? Thank-you.
Yes, Brahman is the ultimate cause of everything in Vedanta. Kenopanishad starts with beautiful questioning regarding this matter:
केनेषितं पतति प्रेषितं मनः केन प्राणः प्रथमः प्रैति युक्तः । केनेषितां वाचमिमां वदन्ति चक्षुः श्रोत्रं क उ देवो युनक्ति ॥ १॥
श्रोत्रस्य श्रोत्रं मनसो मनो यद् वाचो ह वाचं स उ प्राणस्य प्राणः । चक्षुषश्चक्षुरतिमुच्य धीराः प्रेत्यास्माल्लोकादमृता भवन्ति ॥ २॥
न तत्र चक्षुर्गच्छति न वाग्गच्छति नो मनः । न विद्मो न विजानीमो यथैतदनुशिष्यात् ॥ ३॥
अन्यदेव तद्विदितादथो अविदितादधि । इति शुश्रुम पूर्वेषां ये नस्तद्व्याचचक्षिरे ॥ ४॥
I-1. Wished by whom is the mind directed to fall (on its objects)? Directed by whom does the foremost vital air move? By whom is wished this speech which the people utter? Who is the radiant being that unites the eye and the ear (with their objects)?
I-2. Because He is the ear of the ear, the mind of the mind, the speech of speech, the vital air of the vital air, and the eye of the eye, the wise, freeing themselves (from the identity with the senses) and renouncing the world, become immortal.
I-3. The eye does not reach there, nor speech, nor mind, nor do we know (Its mature). Therefore we don’t know how to impart instruction (about It). Distinct indeed is That from the known and distinct from the unknown. Thus have we heard from the ancients who expounded It to us.
I-4. That which is not uttered by speech, that by which the word is expressed, know That alone to be Brahman, and not this (non-Brahman) which is being worshipped.
Then for your question.
Why is our body undergoes childhood, young and old stages and finally we have to die?
We get body due to past life actions. Had we get body of gods (devatas) then we won't suffer from oldness (but still we would have death of that body). Chandogya Upanishad states:
तद्य इह रमणीयचरणा अभ्याशो ह यत्ते रमणीयां योनिमापद्येरन्ब्राह्मणयोनिं वा क्षत्रिययोनिं वा वैश्ययोनिं वाथ य इह कपूयचरणा अभ्याशो ह यत्ते कपूयां योनिमापद्येरञ्श्वयोनिं वा सूकरयोनिं वा चण्डालयोनिं वा ॥ ५.१०.७॥
V-x-7: Among them, those who have good residual results of action here (earned in this world and left as residue after the enjoyment in the region of the moon), quickly reach a good womb, the womb of a Brahmana, or of a Kshatriya or of a Vaisya. But those who have bad residual results of action quickly reach an evil womb, the womb of a dog or of a hog or of a Chandala.
So, it is past life action which determines in which type of body we are now. However there are great saints like Markandeya who have stopped ageing even in human body also. It is also related with their Karmas.
Regarding misery of Samsara, actually there is no misery at all (according to Advaita). It is just perceived due to Ignorance:
There is neither dissolution nor creation, none in bondage and none practicing disciplines. There is none seeking Liberation and none liberated. This is the absolute truth. [Gaudapada Karika Chapter 2 verse 32]
So, the goal of life (according to Advaita) is just to remove veil of ignorance through Jnanam so that we may see the absolute truth.