If they're age restricted items, like alcohol, tobacco (and possibly related paraphernalia), painkillers or fireworks, it is illegal for an adult to buy the item on the behalf of someone underage.
It is also illegal to knowingly sell those items to an adult who is buying them for someone underage.
It's not a policy thing, it's a legal thing. All shops should do the same.
In regards to buying lighter fluid though, that may be law or shop policy depending on where you live. Here in the UK, lighters, fuel, cigarette papers and related items are all age-restricted as being tobacco paraphernalia.