There are six persons, P, Q, R, S, T and U. T is older than no one. S is older than P. Q is younger than P. U is older than T but younger t…

Explanation

T is older than no one, so T is the youngest. U is older than T but younger than Q, and Q is younger than P. S is older than P and R. The order from the youngest is T, U, then Q, so the third youngest is Q.