The crucial experiment is this.
Offer a bulky and boggy bun to the suspected individual just ten minutes before dinner.

If this is eagerly accepted and devoured, the fact of youth is established.

If the subject of the question starts back and expresses surprise and incredulity, as if you could not possibly be in earnest, the fact of maturity is no less clear.
-- Excuse me,--I return to my story of the Commons-table .-- Young fellows being always hungry, and tea and dry toast being the meagre fare of the evening meal, it was a trick of some of the Boys to impale a slice of meat upon a fork, at dinner-time, and stick the fork holding it beneath the table, so that they could get it at tea-time.

The dragons that guarded this table of the Hesperides found out the trick at last, and kept a sharp look-out for missing forks;--they knew where to find one, if it was not in its place .-- Now the odd thing was, that, after waiting so many years to hear of this college trick, I should hear it mentioned a second time within the same twenty-four hours by a college youth of the present generation.

Strange, but true.