This is a French saying and it means that is far easier to start off being strict with children and then get less strict as they get older, than it is to start off very unstrict when children are young and then have to try and get stricter when they are older.
I see it so many times with people I know. They are very relaxed when the children are young, because as long as the children are not feral, you can get away without being strict. But once children get a bit older the "naughtiness" they can do becomes far more serious. So parents find they have to be stricter about rules. But trying to get stricter when children are older is very hard to do.
I am not saying it is easy, but I think if you are stricter when they are younger and teach them what is okay and what is not okay, then when they get older they are more likely to avoid doing things which are really wrong.