That's a good point, the US population of people 18 years or old is ~252 million, so if you only gave people 18+ a UBI handout, you'd cut your costs to 7.862 Trillion dollars per year.
Also one could remove the UBI part already covered by pensions -- e.g. the millions of government employees getting X pension money, can get (X - UBI) + UBI (so the same as they already get).
A UBI that pays a portion of a living wage might be a very good idea, but it's not going to allow the majority of Americans to lay on their butts all day watching TV and eating cheesy poofs.
