Yep its true. Prophet Isa(as) did testify
" She made a sign to them, pointing to the child. But they replied : 'How can we speak with a baby in the cradle?' Whereupon he spoke and said : I am the Servant of God" (19:29-30, Koran)
I think, 'How can we speak with a baby in the cradle?' is a sarcastic remark on part of Jews to Jesus (as) and doesn't testify of him being actually a baby in the cradle. And making such sarcastic remarks are very common for people who are full of self esteem, pride and ego. Otherwise, in the following versus, the message he (Jesus (as)) is giving to them (Jews) doesn't make any sense. For example, the following versus say:
[19:31] He said, ‘I am a servant of Allah. He has given me the Book, and made me a Prophet;
[19:32] ‘And He has made me blessed wheresoever I may be, and has enjoined upon me Prayer and almsgiving so long as I live;
[19:33] ‘And *He has made me *dutiful toward my mother, and He has not made me haughty *and *unblessed.
It doesn't make any sense for a child who is in Cradle, who is totally relied upon his mother to feed him, take care of him in her lap, clean him etc, to make such a claim. Specially in verses 19:32-33, how can he be offering Prayers and almsgiving, be dutiful towards his mother at the age of 2 months or so, when at such young age, a person cannot take care of him/herself let alone to take care of his mother.
It is obvious that Jesus was in his young youth age which can be 30 years or so when this event took place.