I'm not sure about the age thing, but I can speak to the kids part. Becoming involved with someone with children has loads of potential pitfalls, but just as many potential rewards.
When I was younger and far more naive, I was involved for two years with a girl who had a son. He was only a couple months old when we started dating. We were young, starry-eyed, and I at least had swallowed all the "love conquers all" BS that pop culture abounds with. In short, I believed that we were going to be together forever and I gave my heart to both her and her boy. When I was away at basic training, she was unfaithful and the long and short of it was that when our relationship fell apart, I lost him as well. The last time I ever saw him he was fast asleep, and the last time I ever spoke to him was when I was stuck in basic training, over a pay phone. I stood there listening, knowing that I was losing the most important part of my life, and all he would say over and over again was "Home Dad!" That was back in the dark ages of 1986, nearly twenty years ago, but it was the darkest and most painful time in my life, and it still hurts like hell to think about.
It was over ten years later that I dated a woman with children again. I was older but no wiser really. Despite thinking in the back of my mind that I should really be careful about this sort of thing, I fell in love with the kids as easily as with the woman I was dating. Just couldn't help it really. They were both girls, 3 and 7 at the time. They were 4 and 8 when I married my wife, and they're 11 and 15 now. I love all three of them with all my heart and they have given me more happiness than anything else in my life, hands down. Nothing comes close to being a father and husband.
The fact that you are scared is probably a very good thing if it makes you cautious in approaching this relationship. You are opening yourself up to a whole different set of possibilities when becoming involved with someone who has a child, and not just for yourself. Your actions may have a profound impact on the life of that child. Only you know whether you can or should be involved with the child, and just how deeply. Are you willing to be a father if things go right with this woman? How much are you willing to give of your heart and what will you do if things then fall apart? Only you can answer these questions.
The only solid advice I can give is that if you or the mother aren't capable of placing the child's well-being right at the center of things you should walk away now. That child is much more vulnerable and far more powerless than either you or she, and your actions can have a profound effect on a young life.