The SCUMM Actors Guild Card
, that three years would more likely explain the time between MI1 and MI2, as time has obviously past quite a bit between those games, Stan says the time between MI2 and CMI is Three Months "The guy who locked me in my own coffin for Three Months!" and the time between CMI and EMI is also Three Months (as mentioned by the catapult operator about why Elaine was declared dead).
Taking into account his 19 slip in MI2, Three Months pass to make him 20 in CMI, another Three Months in EMI makes him nearly 21 in a way. That would make Guybrush just 16 years old in MI1!