Xiao Wang: 2+L

Xiao Zhang: 2+L

Xiao Yang: (2+ 1)+4

Then (2+L)+(2+L)+(2+L)+4+L=90.

4L+ 10=90

L=20

Results:

Xiao Li: 20; Xiao Wang: 22; Xiao Zhang: 22; Xiao Yang: 26 years old.