If you're in a long-running campaign or if a lot of time in your world passes between adventures, here are a couple of ideas from different games you can adapt to help with time keeping for your PCs.
Dragonbane: Age is listed as young, adult, or old. Actual age isn't recorded in the core rules.
The One Ring: At the end of a calendar year (game time), all characters note that they've aged a year. A character's birthday doesn't have to be recorded.
Assuming beginning characters start young, perhaps a young human will be come an adult after 10 years and old after an additional 30 years. Perhaps the year can be described as seasons rather than actual months or dates unless something noteworthy happens on that month or day.