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.
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).