From what I personally have seen:
in MI2, Guybrush is 19. Since Largo's been on the track of the mystical beard of LeChuck for "years", Guybrush was probably 16 in MI1 (although allegedly the game manuals put just a month between the two games).
In CMI, I gather Guybrush claims 20, consistent with other indications in the game (eg Wallys subscription to LeChuck's pirating course) that a year has passed snce MI2. However, in TMI Stan says it was only three months, and Elaine says it was three years.
EMI is meant to be three months after CMI, but obviously that's not true. Obvoiusly Guybrush and Elaine postponed their three-month honeymoon in order to settle down living together in the mansion, get Timmy, move Guybrush's stuff into the mansion, rebuild the interior of the mansion, move the SCUMM bar.....
OK, but let's say he's still 20.
A 'decade' of adventures in TMI seems reasonable to date from Elaine giving up all her political appointments to become a full-tiime pirate at the end of EMI - so, TMI must be tennish years later, Guybrush about 30 years old