i don't know about any concrete examples other than those already mentioned, but (just a theory though) i've heard a common way for people to fake their ages is to make them "early birthday" kids
the korean school year begins in march, so a kid born in january or february has the choice to either go to school in the same year as other kids born the same year as them... or be an "early birthday" kid, and go to school with kids born the year before them. so someone born in say january 1995 could have gone to school with other 95ers, or with 94ers. now, thing is, if you are batchmates with 94ers, it would be kinda awkward to see them as older. in a social sense, you're kinda treated as one of them. so if our january 1995 born person went to school with 94ers, someone born in say july 1995 would typically use honorifics with the january 95er, as they would with any 94er. so if one was to make an idol seem younger, but the others around them (say, the groupmates) have the habit of calling them unnie/hyung, a convenient way to do that without forcing their groupmates to change habits is to change their birthday to jan/feb of the year following their actual birth year
as i said, this is just a theory - and tbh idk how much validity this has, because management can be pretty powerful in getting idols to change their habits. take for example pentagon's maknae line - all 98ers. they are all actually very close in age (yuto is 4 days older than kino, kino is 4 days older than wooseok), but while yuto and wooseok went to school with 98ers, kino went to school with 97ers. so predebut both wooseok and yuto (who, mind you, is 4 days older) called kino hyung, until the staff made them stop just before their debut because it was too silly.