HI, it depends on the cause of the low sperm count but yes, it is normal to vary based on what is going on in your body, hormones and stress and vitamin levels. The quality also matters. As you age there is more genetic errors made in the sperm but staying healthy is the best way to increase sperm count and quality hope this helps.

It depends on the cause. Some are reversible. For example, if it is related to varicocele and treatment was done people recover.