When a twig grows hard, it is difficult to twist it.
A person can be moulded when young but not when he has grown up. So, one should inculcate good values in children while they’re still young.
Example: Your son doesn’t take responsibility when he fails at something. You must take corrective action now, for when a twig grows hard, it is difficult to twist it.
Phrasal Verb:
dive into [x]
to eagerly begin a pursuit or activity