A couple of things that had helped me that come to mind:
-growing facial hair/stubble/scruff etc. that makes you look older
-putting on muscle weight to look older (depends on your height and weight how much you might want to or need to put on to look older, but if you are fairly lean, muscle will definitely help)
-hanging around guys a few years older than me and who were good with women too, helped me become more like them, and appear more like them to other women
-spending time with older girl friends/time in relationships with older girls to get used to and more calibrated to their type
Age Barriers - it's all about how tight your fundamentals are. Even if you're 18 - if you can show her you think like you're 21 or older and have the "experience" to back it up then age really makes no difference.
As you know, I'm only 19 but I've slept with girls from ages 20 to 29 and a lot of the time I get complimented on how "mature," "wise," or "experienced" I am.
Think about the implications of "I'm too old for you?" What is she actually saying here? I won't tell you because it's something you should figure out for yourself and then you'll see the bigger picture.
Zphix is spot on; much of the time girls telling you that you're too young is just their rationalization (to themselves and to you) for you messing up in fundamentals or in your process. Girls should be able to see the maturity and the edge just in your eyes and know that you are different from other guys and that you mean business.
Oh and the article Zac refers to, you should read as well; Chase discusses how to and why you should lie about your age and a few other touches to getting older women as a younger guy.
