Hi Anent,
I had the same question too..but i think there is no age limit to learn anything..but considering Carnatic vocal there might be certainly some restrictions....If you have a good voice you can train it and develop proper breath control....but then good voice is a gift...unfortunately i dont think i have it..so if u have good voice u can certainly practive and make it better..Or else learn and appreciate carnatic and sing for yourself and feel the emotions that this music supposed to emote(like me)
Good luck
PS: I am not a carnatic artist..just an ardent fan..
Regards
Ganesh