Hmmm... you seem to be making just about every mistake possible. Sorry, don't mean to be disrespectful in that but this can be difficult to grasp when new (as I still am). Apart from what's already been said, you must have some VERY old a/c if the AVERAGE is 21yrs and you have A330ss A320s 757s etc. If what you're saying's for real you need to be more patient, stick to a MAX of 3 fleet-types and wait for better a/c to come along (which they sometimes do, honest!)
Oh, and how can you consider the 737/757 and A320/A330 as "the same aircraft type"?
Read some of the "newbie" advice threads which are very good and then start again while there's plenty of time left in the game. There were just a bunch of nearly new 737's for sale but they're gone now. I'm sure more will pop up at some point.
One thing I'd like to know though... how can you have 4 bases open just 2.5yrs into the game? I think you mean you fly to 4 destinations, yes? If so, you're doing that right - sticking to a low number of destinations saves on staff costs