Premature birth; how to tell if one is affected by it?

I just found out that a friend of mine was born prematurely.—> at 7 months. I haven’t known this friend for long, in fact, I just met this person a few months ago.

Now, I know there can be long term, or short term affects on people who are born prematurely, but at what age can you be certain that they are not suffering from anything relating to being born prematurely? Such as mental disorders or learning disabilities?

My friend seems perfectly fine! How can I tell without asking? I’m just curious, I’m not trying to be judgmental either. It’s just that I also have no background knowledge on this, and would like to find out more.

Friendly and helpful comments are appreciated! =D