I don't think it's the single mom thing. I tend to think that moms of babies / toddlers just don't have the time to maintain friendships. Don't assume that your "old friends" are all still hanging out together without you - they may not be hanging out at all.
Check out your local churches - many have MOPS groups (mothers of preschoolers) - our church has a ladies bible study and includes a nursery for the kids to go to. The bible studies are always based around practical stuff that we need to work on - relationships, parenting, etc so it's a great way to meet other moms. Check out the schedule at your local library for mom & child activities - they usually have free or low cost activities designed for moms & kids of different ages - including toddlers. If there's a YMCA or other community center check those out too. Nieghborhood playgrounds are also a great place to meet other moms.
Finally - if you really feel that you've lost some of your friends do a little sould searching - maybe you're not realizing something about yourself: are you a little flirty with their husbands/boyfriends? are you a complainer? are you in the phase where you like to go out and they aren't in that mode any longer? Are there differences in finances - such as they have the resources to go out for lunch while you can only brown bag it? Is there an unevenness in your babysitting trade-offs?
My best friend in the world is 10 years younger than me. So age generally doesn't matter much. But an observation based on what I've noticed on this site is that many (NOT ALL) younger moms are still pretty childish - still acting like teenagers with attitudes and adolescent insecurities - instead of mature parents. I'm not sure why - maybe becuase alot of moms aren't married these days and don't have that kind of security so they ARE still insecure? But all of that may play into the relationship thing that you're dealing with.
Finally - pray about it - ask God to bring you a good friend who you can relate to with a kid your kids' age. Sounds silly - but I truly believe that we have a God who cares about the details of our lives. I prayed for a best friend - and within a few weeks my best buddy and I found eachother - at a group our daughters belonged to at church. But I went out of my comfort zone and asked her to get together with the kids another time. Ends up her son was also the same age as mine - so we've had many years of being pals and sharing babysitting, sleepovers, childrearing woes, day trips during school vacations, etc. Now we are raising teens and we have a whole new set of issues to share! We've been there for eachother through deaths in our families, cancer, spouse's injuries and illnesses - and it's been amazing.
Good Luck A. - there is a group of friends out there for you. I know it. ;o)