You can easily buy BTC off ebay or make them by selling stuff or partaking in a signature deal.
They mine. Buying Btc is not the only way to get it you know. Some might have done it long time ago, others mine alt coins and exchange for btc. They won't be able to change those btc to fiat until they are 18 that is for sure.
How would under 18's with no jobs be able to afford mining equipment that sounds a little unrealistic to me.
You can mine some altcoins on laptops or standard computers. Under 18s can also have jobs.