Are you allowed to put a child in a booster seat with a sash belt?

It shouldn't go on age, it should go on weight and height but IMO the longer you leave them in a proper carseat the better.
Most carseats go to 18kg.

Depends on the Booster seat. Mine was able to be used from 9 kg, so for quite young kids, although my son was 4 by the time i got it. The ones that have seatbelt guides inside them (not just the velcro on the side) are good. He grew out of it at 7, and is now in a half booster still at 8.5!

We've just started using the sash belt now at 4.

As above - no particular age but when your child outgrows their carseat.

I initially had Carlos in a booster with a harness inside it as that is much safer than a booster with a sash belt. I would keep them in a harness whether in a carseat or in a booster as long as possible as its just so much safer

It's a weight and height thing... child should be over 18kg's but also something like 114cm (I think) before going out of a 5 point harness... if your child hits 18kg's before 114cm then you can buy those H belts. DD1 is 4.5 and she is still in a 5 point harness and will be for a while. It is just soooooooo much safer. You just have to You tube 5 point harness and see why.

I still have my 4.5 year old in a 5 point harness aswell. He is 17kg and tallish but still feel its so much safer.

My booster was 14kg, as thats the weight that a re-tractable seatbelt will work.

It meant my girls were nearly 4 when they went into one, though they started to climb out of there carseats, so it was safer for them to go into a booster and tell them they were big girls.