Why do toenails get thicker?

Before he died, my dad spent some time in an assisted living center. Part of his regular routine was to get his toenails cut, which I noticed had gotten so thick that they had to use trimmers that sort of looked like side cutters.
As I get older, I keep checking my toenails, looking for evidence of increasing thickness, density and resistance to normal trimming.
Is this a normal feature of aging, or was it just something my dad experienced because of some other systemic problem.
Should I buy some side cutters?