This is not about maturity, but about different stages of life. A 22-year-old who is old enough to have finished university and even a Master's is further along their life journey than a 17 years old A-level or equivalent student and has other priorities in life. At 17, you almost 100% still live with your parents, most likely have not had a job, may not even know what degree to pursue if you want one, and are still likely to change once you reach university (in fact, you may find yourself falling in love with the cute guy who sits next to you in some of your university lectures). On the flip side, a 22-year-old has already dealt with these things and is now focusing on their career and on finding a long-term partner. 17-year-olds and 22-year-old have different priorities in life, as exemplified above, which could result into plenty of conflict and disagreements. No matter how much you want it, a relationship you enter at the age of 17 is quite unlikely to succeed, which is not what a lot of 22-year-olds look for (those who do not look for it are most likely ***boys or not ready for any commitment you want from 'mature' guys).
Besides, there is the issue of what kind of adult would agree to be in a relationship with a teenager. I am 18 and I would not be in a relationship with a 17-year-old since I am an adult and they are not. If an 18-year-old would not date a 17-year-old, what kind of 22-year-old would? Those who think you are naive and innocent (basically, they think of you as an easy f***).
Not saying all relationships like this are doomed to fail, just that they often do because of things outside of your control, so I personally would keep the age difference as small as possible.
That being said, this becomes less and less of an issue as you grow up. It certainly will not be an issue when you will be 43 and 48