I started at a 12-13 average (I live in a hilly area), got up to about 15.5-16.5 after about 3 years. Then, about 2 years after that I got sick, lost a lot of my strength and took about 1.5 years to get back somewhat back to where I was. But, my attitude has changed. Aging has also kicked in. I need more time to recover and frankly, I just don't care about my speed anymore. My goal is to ride until I croak, and I know that I am not going to keep getting faster at age almost 59 unless I put in some serious training. I am still too fast for "slow" groups and too slow for fast groups. I found one group that I can ride with, and now they often split into the regular group, who averages about 13 on rides and the "spirited" group who averages about 15-16. I am on the edge of that and they are encouraging, but I just don't ride much with the group anymore.
I do short rides (15 miles) where I push the speed, and my hilly riding serves as intervals. Other than that, I don't care. Today I rode 43 miles, with several very steep climbs. Until the lunch stop, my average was 12.5. After lunch, we had a long flat stretch where we were going along at 18-21, so my average was 13.8 for the ride, that is, until my cool down, which involves climbing a 10% grade hill to my house.
I can go along at 18-20 mph if I am on flats, but the thing is, there is no place around here to ride that is flat. I can also climb better than a lot of women my age, and I am not afraid to spin up a hill in a very low gear. I don't like descending, so my average suffers because of that.
Cyclists get very competitive about their speed, etc. I found people to ride with who just aren't into that. They are great riders, but not particularly competitive. This is why you will never see me doing a tri or even a charity ride where some people make it into a race.