With the law and paintball there are a few heads in the sand.
The law is black and white, there are a couple of grey areas but that is only subject to where things fall in the definitions. Interpreting the law is always subject to an individuals opinion and tested in law when in court - then it's the interpretation of the judge based on legislation and evidence presented
For kids it is legal for them to use paintball guns on sites if the interpretation of a club/firing range under adult supervision is considered to include a paintball site. There are plenty sites with people below 17 or 14 playing so if they are legal then that is within the exclusion and the age becomes irrelevant which would make 8 year olds legal
Steve Bull has spent years putting the case forward, sometimes people are happy take what he says or to disregard his recommendations if it suits them
A couple of explicitly illegal areas could be:
Reball sites. Reballs are not frangible, this could be taken to say its no longer an 'air weapon with frangible ammunition' and then push it to a licence-able firearm - and there is not an appropriate licence, but you could try applying at your local police station
And full auto/burst fire. I own a number of full auto capable paintball guns. It's generally accepted that though many paintball guns made for the uk have modes removed from boards that some with smaller production runs, early models or aftermarket boards are legal to sell, buy and possess as long as you don't use them but the law states 'capable'
A recent law change that is often ignored is the requirement to lock away air weapons if minors could have access to the property. This follows a child who died in an airgun accident.
This comes with a £1000 fine. In the legislation it just needs to be locked away, Police & home office recommendations want people to use firearm safes, and also recommend they are locked in the property and not sheds. If your shed is broken into and paintball gun stolen you could find yourself being fined if the police decide it was not secure enough