The generally agreed limit (also established in court cases) is the amount of pocket money a child of the given age typically gets per month. For a 10 year old, that's about 20 EUR, for a 16 year old about 50 EUR. A console would definitely be too expensive, as would be big name concert tickets. Unless it's a recent AAA title, video games would be OK. No idea what Warhammer costs these days.

Most retailers are probably willing to take the risk of maybe having to do a refund, unless it's something really expensive (or perishable/consumable).

There are definitely limits in some countries relative to the US. I was in university at 16. My parents were covering a lot of costs but I was certainly making regular purchases of all manner of things. My understanding is that would perhaps be something of an issue some places.