This is a long shot but it's possible you're conflating two different animated series, which I recall often aired together around that time period in the US. (though you didn't say where you saw this) This was suggested in comments by SPArchaeologist and MarbleMunkey.
Within the combined and edited story, Robotechnology refers to the scientific advances discovered in an alien starship that crashed on a South Pacific island. With this technology, Earth developed giant robotic machines or mecha (many of which were capable of transforming into vehicles) to fight three successive extraterrestrial invasions.
Source: Wikipedia, Robotech (TV Series)
Specifically the SDF-1 Macross can transform between spaceship and giant robot form.
The first episode featured something like "a baby boy who aged over night into an adult".
...Lion-O is now a grown-up ThunderCat. His sleep-chamber was damaged and his body grew many years older while the others remained in suspended animation. He is now no longer a Thunderkitten ...despite still having the mind of a child and lacking the experiences that would make him a good leader.
Source: Thundercats wiki, Exodus
An infant version of Lion-O was also featured in the episode "Time Switch". (and also in the 2011 remake of Thundercats)
Both series originally aired in 1985. In 1998 Thundercats and Robotech aired as part of the same lineup on Toonami (Cartoon Network). Here's a promo. Even if you didn't see it on this channel, it's very possible you would have seen them together.