Micropenis is typically noticed in the initial physical exam after birth. The upper limit for that criteria is about half of the typical/normative, so it's visually distinct even at birth. It's not determined by erect length at all.

Penises do grow over time, no matter what size they start, but there's limits to how much. Even as puberty hits, someone with a micropenis will only get so much extra because of the underlying limitations of the tissues. If someone of normative length gets a 10% change (as an example, I don't recall the actual number ranges for pubertal changes), that same basic range is all the person with a micropenis is going to get too.

And you're exactly right, it has next to zero impact on long term relationships. I wanna say that out of maybe fifteen or sixteen patients I had that fit the criteria, all but three had kids. So it's definitely not a barrier to sex at all. The one patient I had that was unusually talkative about it (most of them would just state the fact and describe any special needs they'd have for bathing, then never mention it again) said that once he read "the joy of sex" and learned how to do oral, he and his wife did fine, which she said was true as well, fwiw.