Colossal Titan is much much bigger than the Armored Titan, yet both of them are of the same or at least similar age.
As another comparison, Jaw Titan is of the same if not older age than Eren yet much much smaller.
So there is no relationship between their age and their size.
Another example Connie's mother. Despite being old, she didn't turn into a huge titan. In fact she only became a small titan.