The cost of an annuity that pays your basic, fundamental living expenses: Food, shelter, water, etc. can be thought of as a debt that all people are born with. In the US, you can obtain a room in an apartment, basic food, internet access, and transportation (minimal versions of all of these), for something like $15,000/year.

such a lifetime annuity would cost the average 18 year old on the order of 200-300 thousand dollars. This means that an 18 year old starts with about the same level of implicit debt, that a doctor who has just finished medical school has explicit debt, except that student debt doesn't scale with inflation.

A key feature of a society which values social mobility is reducing the debt burden of its children. I don't think the answer on basic income is cut and dry, and I do think its important for people to sustain themselves via their own efforts, but since you likely wouldn't increase the debt burden of a teenager, and you also didn't select the debt burden of the average teenager. You should at least consider the possibility that you would actually choose to reduce, by some amount, the debt burden of the average teenager.