I hate hearing stories like this. I hate it because - particularly with big companies - they'll have a system that lets them get away with such things. They'll always be just within the letter of the law. If they want to get rid of you, they'll find a way. It pisses me off something major.
If I might make a suggestion: checking with a labor lawyer or state employment agency to see if they are descriminating is a good idea, but don't hold your breath that anything can be done at this point - you're still working, after all.
But if you want to stick it to them just a touch, something you can do is make sure you toe the company line. Dig out your employee handbook. Play their game and see if you can't beat them at it. If there's a definitive procedure for asking for time off, make very sure you put that slip of paper in the right box -
after you make two photocopies of it: one for your supervisor, and one for yourself. With the date you submitted it written on it. After all, you certainly don't want this kind of misunderstanding to jeopardize your future employment with this fine company, do you?

It might not make any difference in the way they treat you, but if you are a model employee it makes it hard for them to find a reason to fire you. So if they do then you can claim unemployement benefits and maybe even go after them for unlawful discharge - they have to pay back wages and maybe even penalties for that. And if you photocopy all your internal transactions you'll have a paper trail your lawyer can use.
Also - most corporations have a separate HR department - and the HR department in addition to doing the hiring and firing also deals with employee discipline issues - and working conditions. Find out the person who heads up the whole HR Department - the HR Manager, the HR Director, the VP of Human Resources, whatever - and if this behavior continues drop that person a little note saying you believe you are being discriminated against because of your age (or some other factor not known to you). Hit the high points, don't go into too much detail, but name names. Do not threaten to go to the authorities, but make sure you keep a copy of whatever you send them - and don't do it by phone, do it on paper or e-mail (print out the e-mail) so you can prove you went through proper channels before you sue.
And as you do this - have you considered temping? Depending on your skills, a temp/staffing agency might be something to think about. Depending on the market, you can tailor the jobs you accept to meet your needs. Of course, there are a lot of variables in that equation but if you need to work special hours and you can be a little flexible on income, that might be an avenue.
A staffing agency might also be a good idea for helping you hunt for a new job. Just avoid any that want you to pay.
Sorry for the rant-like nature of this - but I've been there and I have little faith in depending on justice at the hands of others.
Stay warm!
I am
Dawg