Johnny Cade is 16-years old. He is small for his age so he actually looks to be closer in age to Ponyboy, who has recently turned 14.
In terms of maturity, there are moments in the story where it seems Johnny is treated as if he is much younger than the rest of the gang. Ponyboy initially refers to Johnny as the group's "pet". He compares Johnny to " a little dark puppy that has been kicked too many times." The group is very protective of Johnny, similar to the way one would be to a smaller kid that cannot fend for himself.
After Bob's murder, the Boy's go into hiding and it seems as if the roles have reversed. Johnny appears to be the stronger one. He is the one who suggests going to Dally for help, he comes up with the idea to cut their hair, opting to bleach ponyboy's and wash the grease out of his. Johnny is also the first to suggest going back home and turning himself in.
Things come full circle when the church catches fire. Johnny has always been the meek one; the one in the group that needs protection. But when he and Ponyboy realize that there are children in the church, Johnny springs into action. He doesn't hesitate. Ponyboy notices the change. He says:
"Johnny wasn't behaving at all like his old self...he wasn't scared either. That was the only time I can think of when I saw him without that defeated, suspicioius look in his eyes."