OK, since you used Turtwig as an example, I will too :P
Turtwig learns Leaf Storm at level 45, so you are wondering how a level 5 one could know it too. Here's how:
- Teach a Pokemon in Turtwig's Egg Group Leaf Storm (this Pokemon must be a male), at whatever method/level it must be. Turtwig and it's parents all are in the Grass Egg Group and all can learn Leaf Storm, so we will breed with them.
- Have a female Turtwig, Torterra, or Grotle go in the daycare with the Male knowing the moves. The feamle must know the moves identical to the male that you want passed to the Egg Turtwig, if they are learnt by level up.
- Once they have bred, you will get an Egg. The Egg will know some of the moves the father knew if you have done it right (See here), in this case Leaf Storm.
The main purpose of breeding is usually to obtain a Pokemon that knows certain moves. If the male Pokemon knows moves that the baby Pokemon is capable of learning, it will know them when it hatches from the egg. This is a good way to ‘reuse’ TMs that you taught a Pokemon and cannot re-obtain in another way like buying.
The baby will know any move that it learns at level 1. If both parents know a move that the child would learn by level up, the child will also know it upon hatching.
Furthermore, there are some moves Pokemon can only learn by breeding - these are called egg moves, and are listed in our pokedex alongside the other moves. In most cases, a Pokemon in the same egg group learns the move by level up and can pass it on by breeding, but sometimes you need to ‘chain breed’ from one Pokemon to another to another.
If there are too many candidate moves that the baby can learn, they follow this precedent, with each new move overwriting previous ones:
1. Level 1 moves.
2. Moves that the child learns by level up, if both parents have them.
3. Any compatible TMs, HMs and move tutor moves known by the father.
4. Any egg moves known by the father.
The above copied from here.