Correção de texto "The last thing is the influence..."

Pessoal, eu estava escrevendo um texto, e coloquei essa frase, meu professor falou que as preposições em negrito estão erradas, como posso melhorar a frase? Achei q a frase ficou meio estranha, alguém tem ideia de melhorar ela, não só mudando as preposições e tal...

The last thing is the influence of my elementary school. Most of my classmates didn’t want to go to college, and they stopped on the high school. On the other hand, they were parents with thirteen, fourteen years old. That is the example that I didn’t want to follow.

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Gabriel, you could try this:

The last thing is the influence of elementary school (no pronoun needed). Most of my classmates didn’t want to go to college, and they stopped in high school (the term "high school" takes the preposition "in" and no article). On the other hand, they were parents at the age of thirteen or fourteen. That is the example that I didn’t want to follow.

In English, we are an age and we can be at an age. We cannot be with an age though.