Marrying is marriage.
In a marriage there are a lot of things to consider. It only looks like only the two of you are involved, but many people are involved. For starters your parents, then her parents, your relatives, your friends, your colleagues, your social life. Ditto on her side. They are all involved in your lives, unless of course you isolate yourself. Also consider schooling, community life, leasure, housing, etc.
You can wait, till you are stable and earning a steady income, she cannot.
Sex is not a reason for marrying.
- Sorry double post -
Ask yourself do you really love her? Would you accept her looks in 10 yrs time? Ask yourself is she the one you are really going to spend your life with her? If the answer is Yes for all the answers then you can start to think about how to influence your parents thinking about dating an older gal.
Not doubting you but you should try and address the your parent's issue if you would want o settle down with her.
I think your parents' have their hesitation as they fear that you might be rash with your decision. I think if the both of you truly love each other, I am sure she would be able to prove to your parents and alay their fears and concern.
You just need to be proactive here. Afterall, you have responsibilities to your parents as well - for they have brought you up for what you are today.