Who volunteers? This is a large question that you need to look at from a couple of different views, I'll try to not make this too long but its important.
First look at the profile of volunteers over all in your area. Try starting here. Do two things, look at the percentages by age groups and then look at your volunteers. Look at your advertising material. Do they match? Does the demographic match what you see? If so, you aren't done. If not you have some work to do.
You should look at the type of volunteers you want (age, skills, time to give) and go get them! In volunteer management we often recruit in the same old way; because its the way we do it. Remember it's advertising; if you advertise and only show gray hairs (like me) that is what you will attract. Advertise and use photos of a mix of age, race and sex. That is what you want to attract.
Now don't send me hate mail saying I am advocating for not attracting baby boomers and older, that's not the point. The point is many people equate volunteer with older Americans! Yes the retiree has more time to volunteer, and they have a great amount of experience. But who will they pass the knowledge to?
And if you are looking for sand baggers for flood season, AARP may not be the group you want to focus on.
So what does your city profile look like?