Right now, if a world starts in 1990 and ends in 2020, even if somebody starts an airline in the final year, they start with an MD80/737 classic. Of course this doesn’t make any sense, so I suggest basing the starting aircraft on the time an airline joins.
Example:
1990 to 2000: MD80/737 classic
2001 to 2015: A320ceo/737 ng
2016-2020: A320neo/737 max