I would say if the main pairing, main characters are over 50, it's fine. Of course there will be kids and grandkids running around! That's encouraged, but, for example, having a fic where Scorpius and Teddy are the main pairing, but are in their 20's and it just happens to have their parents in it (in their 50's), that belongs somewhere else.

If the main pairing is Neville and Draco in their 50's or 60's, and their grandkids are around... no problem there. ^_^