Under 5s or under 5’s.

[Peppapog263]

I was always under the impression that this didn’t need an apostrophe but twice yesterday I’ve seen it with one. Once at a shopping centre ‘a fun session for under 5’s…’ and then at a playgroup.

[SomeSuchThing]

No apostrophe! :)

[MakemineaGandT]

No apostrophe.

Same as ‘born in the 1960s’ or whatever…no apostrophe needed there either, yet you often see ‘1960’s’

[Peppapog263]

Thanks, I was sure it didn’t need one but I keep seeing it everywhere lately!

[upinaballoon]

I'm in my 70s and there was a time, when I was younger, when I would have put the apostrophe. Now I wouldn't. It might depend on the time we were educated formally. I think of myself as fairly fixed but I moved on that one.

[Abouttimemum]

No apostrophe.
If you write under fives (rather than the number) you will see that it requires no apostrophe. It doesn’t belong to anything.