I think she might have seen a lot of her peers marry early with hardly any qualifications and then got divorced thus plummeting their quality of their life. I'm assuming you're from a two parent household, so your hooyo probably thinks that the reason why it worked out for her is because she married later and therefore was able to make better decisions.
My advice would be sit her down and ask her why she thinks you're not ready for marriage. It might be a hard pill to swallow, but she might think you're not mature yet, or ready for the responsibilities of being wife or even mother.
. OR she might be afraid that if you meet someone you'll give up education and work which was the norm for her generation. Remind her that things have changed and you can get to know people whilst building your life.