What makes a person have strabismus (crossed eyes)?

Need for glasses +/-: When eyes cross it can be due to an excessive amount of hyperopia (farsightedness), but many times it is because that individual just has a tendency for the eyes to not stay aligned all the time. While this isn't normal, it's not rare. Other things such as muscle palsies or trauma can also cause this but these are less common.

Depends on the age.: Depends on the age of onset. Develops at birth or shortly thereafer, or around the age of three and associated with far sighted refractive error, or associated with an heterophoria (strabismus controlled by patient and only recognized by examination) that has decompensated into a classical strabismus, or from trauma, neurological diseases, certain medical conditions, or stroke to mentionfewcauses