Can brain atrophy be normal in a 42 year old?

No: Our brains shrink as we age from 30 to 90, but this is due cell size reducing. The term "atrophy", if used correctly, means the degree of shrinkage is more than normal for age. This is generally an imaging term, like from MRI data. Atrophy can then be graded mild, moderate or severe and given regional specificity (e.g. Global v. Medial temporal). This then means a loss of brain cells.

Yes and no: Depends how you define "normal." Brains atrophy "normally" as we age. This doesn't necessarily correspond with how a brain is working. Elderly atrophied brains can work just fine. Some genetic & degenerative diseases cause atrophy as part of the disease. Occasionally, a younger neurologically normal person's MRI will be read as mildly atrophic by radiological criteria but the person is okay.