The EEOC will share a copy of your complaint with the employer (actually, they've done so already) and ask for them to provide a written response. The EEOC will speak with you and any one else whose name you've provided to them as having direct information in support of your claim.
Then you'll likely wait 12 - 18 months for the EEOC to issue an opinion, which will be whether or not there is probable cause to believe prohibited discrimination to sue. If the EEOC feels there is, there is a slight chance (at best) that they will sue on your behalf. If they don't do that, the EEOC will issue you a "right to sue" letter along with their opinion statement (regardless of whether or not they found probable cause.) At that point, you'll have 30 days to file a civil suit against the employer.
In my personal opinion, I think you're going to have a hard time substantiating a gender and age discrimination complaint. You were 41 when you were hired. If they'd had a problem with your age and/or gender, they never would have hired you. If the schedule change was retaliation for making the FLSA complaint, then the age and gender of the person they hired to replace you is irrelevant.
If you have a complaint to bring, it appears it would be for retaliation for making a FLSA complaint and that is the province of the federal Deparment of Labor, not the EEOC.
A person, who is nice to you, but rude to a waiter, is not a nice person. (This is very important. Pay attention. It never fails.)