all form is finite as well
as of purposeful design,
first being born, then matures
and with /in time decays

shouldn't it be possible for an infant or a kid to remain in his physical form forever?

In theory, it should be possible. In practice, I think it's unlikely.

The requirements for indefinite physical existence are (as outlined in Is it possible to live forever, by manifesting that thought?) :

• Launch a constant stream of new desires
  
  
• Consciously release resistance

While young children are naturally good at releasing resistance (because they still remember what alignment with Source feels like), I think it's the first requirement ("Launch a constant stream of new desires") that they are going to struggle with.

A fresh newly-emerged-from-Source child starts without much physical resistance because it hasn't "banged around in the contrast" long enough to sharpen up its physical focus and decide what it likes or doesn't like - a definite requirement for launching desires, don't you think? :)

It's probably going to take some years of physical life experience to build up to having some reasonably powerful desires, and probably even more years to build up to a constant stream of new desires.

And the irony is, by then, they'll have probably forgotten how to "Consciously release resistance" and will end up posting questions on Inward Quest :)

The other reason I think it's unlikely that a child would remain in that form forever is - and this is perhaps the main overriding reason - it simply wouldn't want to :)

If you're being honest with yourself, would you rather remain forever as a young man or a young child? Generally speaking, children aren't allowed to drive, go to many movies, get involved in intimate relationships, have a career, buy alcohol, commit identity theft etc. etc...the list goes on and on :)

Most children I've known (including me) couldn't wait to grow up quickly enough to get some adult-like independence...so I'm not convinced that even a mature child would want to remain as a child :)

I would say if you truly know your self at a young age then yes you will stay the same. since the outside will not affect you as much, compare to someone that does not know himself and try all is life to find him self on the outside and trying to be like by every one to the detriment of himself.

one should know thyself.

https://www.youtube.com/watch?v=svfDdcPmELk

as for the physical it is still under the law of nature. but if you are less stress and more calm there is some physical benefit to it and you should live longer.

Benefits of Meditation:

The benefits of meditation are manifold because it can reverse your stress response, thereby shielding you from the effects of chronic stress. When practicing meditation, your heart rate and breathing slow down, your blood pressure normalizes, you use oxygen more efficiently, and you sweat less. Your adrenal glands produce less cortisol, your mind ages at a slower rate, and your immune function improves. Your mind also clears and your creativity increases. People who meditate regularly find it easier to give up life-damaging habits like smoking, drinking and drugs. Meditation research is still new, but promising.

http://stress.about.com/od/tensiontamers/p/profilemeditati.htm