It's not about age. If Marcus Rashford is ready for England then he should play.
The science can tell you that the thousands of pseudo-scientific parenting books out there - not to mention the 'Baby Einstein' DVDs and the flash cards and the brain-boosting toys - won't do a thing to make your baby smarter. That's largely because babies are already as smart as they can be; smarter than we are in some ways.
I really like playing other people. There is no other feeling like it, to have a different voice come out of you and to have a different life for a couple of hours. I like being myself. But maybe it's like you ride a bike every day and someone says, 'For two hours tonight do you want to ride this Harley?' You'd be like, 'OK yeah!'