just to ck-i can keep the money there tax free til child is 20-is that right?
Yes. And cash out from 18.
does it just get rolled into a taxed account when they reach 20?
If you don't cash out they become NISA accounts consuming the NISA allocations. See below link.
my children are 19,17 and 13 so is it pointless to do for the older ones?
I started one for my son at age 15 so I could get 4 years invested for him. As an investment time horizon, 5 years is too short but I'm not planning to cash his accounts out to pay for his higher education (Assuming he even goes.). This is more something to help him get started off in life a bit later on. (Although. He doesn't know about the account and I have his Japan Post card. So if he continues to dissapoint this is going to buy me a sportscar. Not saying he's a poor student but I'm down to chosing interior trim!)
But I digress. As with all investments, decide when you need that investment realised and base decisions on that.
Currently you can open J-NISA until the year they are 19 on Jan 1st of.
日本にお住まいの0歳～19歳の方（口座を開設する年の1月1日現在）
And apparently, just to mix things up a bit, from 2023 you can open Adult NISA accounts from Age 18. (Assumedly this is following on from recently allowing them to vote for whichever octogenarian led policitcal party most appeals to them.)
成人年齢の引き下げ（20歳から18歳）により、2023年1月1日以後に開設される口座からは、1月1日時点で18歳である年に一般NISA口座が開設されます。
They can start the paperwork to open
adult accounts from the preceeding November in preparation to start investing from January. (I've personally not managed to open any investment account in less than two months but my mum says I'm "special" so that's probably why..)
And I'm failry sure you can open that account even if you become 20(or 18 as soon may be the case) on Jan 2nd. For "reasons".
Loads of clear and accessible information on this page.
https://www.fsa.go.jp/policy/nisa2/abou ... index.html
Imagine how mad they'd get if they accidently presented something simple and logically.