The FM broadcast band sits just below VHF channel 7, and if your antenna is designed to get VHF reception at all, it's going to get FM. There are a few cases where a specific antenna will have something called an "FM trap" built in that actually stops FM reception. If you don't know if your antenna has one of these, it probably doesn't.
It doesn't matter how old the antenna is, but if it's so old that the wire coming out of it is flat, you may need a transformer
to convert it to coaxial, unless your A/V receiver is also so old that it accepts a flat cable using two screws for the antenna connection.
Most UHF antennas will actually do a decent job of FM reception as well, even though they aren't designed for it. It's just that FM radio doesn't require a very complex antenna for reception. If you have an antenna of any kind up on the roof it's worth giving it a try.
To get FM reception into your A/V receiver, you'll need to split the signal using any simple splitter. If your A/V receiver has an antenna input with two screws (and this would be an older receiver) you'll need a transformer
to change the signal to 300 ohms. Many A/V receivers have a simple push-on connection for an FM antenna, and for this you just need to use any F female "barrel" connector.