Does myopia only happen to adults with weak eyes?

No: Myopia is most likely genetic. There is some evidence that doing a lot of reading or other tasks that require up close vision may also contribute to myopia.

Multifactoral: People with myopia are commonly stated to have "weak eyes" but this is a misnomer. Myopia is a multi-caused change in the optical power of the eye and in fact the eye optically is too strong. Glasses used to correct it are minus lenses which alter the light input so proper focus is possible. It is extremely common, probably has some inherited component but no single cause has been identified.