I can't really why decade birthdays are any different from the others. If humans had evolved with 6 fingers per hand, we'd probably count in duodecimal (powers of 12) - your birthday would then be your 42nd - would it still be significant ?

I do notice that with passage of years my memory is not as good - for example, I tend to forget that I'm in my late 50's (decimal) and thus fail to behave in a manner deemed suitable for my age !