De los 40 para arriba, no te mojes la barriga
 

This saying means that once you turn 40, you have to take care of yourself, and stop doing things that You used to do. Is there an equivalent in English?

Look here.

He's over the hill.
It's time to put away childish things.
It's time to put away the toys.

~~~

There are many of us who are over 40 that are saying,
De los cuarenta para arriba, disfruta de la vida.

Life begins at forty.

[QUOTE=Rusty;42575]Look here.

He's over the hill.
It's time to put away childish things.
It's time to put away the toys.

~~~

and where do you insert 40 in these sentences ?

We don't!
They all infer an age at which you should probably start slowing down. When you're 40, that age becomes 50.
When you're 50, it becomes 60. :D