How the account age is calculated
Reddit supplies a creation timestamp. We subtract it from the time of the lookup and count complete 24-hour periods. The creation date is displayed in UTC so a local timezone does not move it to a different calendar day.
Months use about 30.44 days and years about 365.25 days, so those displays are approximations. Use total days for a closer reading. A missing, invalid, or future timestamp is shown as unavailable, never as a negative age or a valid new account.