Seriously, your best bet (and the advice I would give my 20-something year old self) is to SAVE AS MUCH MONEY AS YOU POSSIBLY CAN. And then save more than that. Try to be financially independent by 50. If you can do that then you won't have to worry about ageism. You'll be able to be picky about the kinds of jobs you take.
From a practical standpoint that means don't buy status symbol products (cars, fancy phones, etc) - stick to practical (I drive a '98 Civic that cost me $2400 - great, reliable car). If you buy a house, buy a very practical one in a more affordable neighborhood. Learn to do your own maintenance. Keep your expenses as low as possible. If you don't have a Roth IRA start one today and begin fully funding it. If your work offers a Roth 401k option put some money in there too if you can.
And of course, always be learning new things.