Step 1: Write each statement as an inequality.
P says "I am younger than S", so \( P < S \).
R says "P is older than me", so \( P > R \), which means \( R < P \).
Q says "I am neither the youngest nor the oldest", so Q is not the smallest and not the largest age among all four.
Step 2: Combine the P and R statements.
From \( R < P \) and \( P < S \) we get \( R < P < S \).
So among P, R, and S, R is already the smallest.
Step 3: Place Q using its own statement.
Q cannot be the youngest overall, so Q cannot sit below R, since that would make Q the youngest, which contradicts Q's own claim.
Q cannot be the oldest overall, so Q cannot sit above S.
So Q sits between R and S, which means \( R < Q \) as well.
Step 4: Check who is youngest.
We now have \( R < P \), \( R < Q \), and \( R < S \), so R is smaller than every other person.
Final Answer:
R has the smallest age among P, Q, R, and S, so R is the youngest.
\[ \boxed{R} \]