So in the new episode, we see Eren's real Titan form. He was a small titan so I was just curious if there's any correlation between the age and the size of the Titan someone would turn into?
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.
In episode 38, released on July 23, 2018 we are shown Eren to be transforming, not very successfully, into a titan as experiments directed by Hange. I do not recall if in the anime this was said, as this part was truncated from how it appeared in the manga, but what happened is that Hange told him to transform many times in succession, and as he got more and more tired from transforming, the titan he produced became smaller and less fully formed. Therefore, I think the explanation for the child titan is that as a child, he does not have the stamina of an adult, and not able to produce his full adult titan form, and not that titans somehow grow with age.
Indeed, as others have mentioned, there seems to be no other correlation of size and age, given that Bernholdt, Reiner, Annie, and Eren are all relatively close in age, and their titans tend to be of various sizes, whereas Connie's mother is significantly older than they, and she became a relatively small and unformed titan.