Seven friends S, T, U, V, W, X and Y, each has a different age. W is older than X but younger than V. Only three people are older than V. T is older than U but younger than S. Y is the youngest among all. How many persons are younger than U?

4

 S, T, U, V, W, X and Y

1) W is older than X but younger than V. Only three people are older than V.

⇒ _ > _ > _ > V > W > X > _

2) Y is the youngest among all. 

⇒ _ > _ > _ > V > W > X > Y

3) T is older than U but younger than S. 

⇒ S > T > U > V > W > X > Y

4 persons are younger than U