let

x = the age of coach John

y = the age of Tim

coach John is 10 years older than Tim

x = 10 + y

when Tim started football 3 years ago, coach John was twice his age

x - 3 = 2(y - 3)

by solving the system of equations

x = 10 + y

x - 3 = 2(y - 3)

we find

x = 23 years

y = 13 years

Tim is 13 years old now.