It's just legal status. For insurance, etc.
I suggest that you take a search for underage, that should show you the reasons and the discussions that have gone on about it.
About you finding a retailer, what do you mean? Do you plan to lie to them?
RS Type 56-1 & RS Type 97B & KWA M11A1 w/steel kit
KJ M1911A1 & KSC G19 + AI Grenade