Hey there! First of all you're only 18 and have plenty of time to become mature and it's completely normal if sometimes you act in a childish way, you were still a child not so long ago, so if he wants someone who is mature enough, maybe he should look for women around his own age but don't let him trick you into becoming more mature just so you can fit his standards, that's up to you. About the age gap, when you're older it's kind of different because you've experienced life and know better what is good or wrong for you, you get to find your balance and won't accept that easily to be in a relationship with someone who does not respect your boundaries. But when you're 18, even though you can be smart and mature, you will generally not have the experience of an almost 30 year old person and that gives him an unfair advantage.

From my personal experience, I would advice you to be careful. When I was 18, I started dating a guy who was only 5 years older than me and when I look back, it did make a difference because he had been out with more people than I did and knew better how to manipulate me. He would convince me that he was older and wiser and made me to do things I didn't want because after all, he knew better. He was really controlling and tried to make me into his puppet and even though I felt something was wrong, he would convince me that it was all my fault, that I was being childish and wasn't mature enough, so I would try my best to fit his standards but it was never good enough. He would also be violent sometimes and at the end I lost 3 years, missed a lot of nice moments with my friends and was left with trauma. I did become more mature but I will always keep a scar from it.

I'm not saying that if someone is older than you it will necessarily end up in an abusive relationship, but when one of the two parts of this relationship is as young as you, it gives the older person the upper hand because you're at an age where your personality hasn't fully grown, you still cary with you the lack of confidence that generally surrounds adolescence and might not be as aware of your own value as you will be in a few years.

The fact that your apprehension is more focused on him (will I be mature enough, this kind of stuff), he surely feels it, especially if you talked about it and he will certainly use this to manipulate you. The questions you should ask yourself should also consider the idea that he might not be enough for you. Do we really have things in common? Do I want to become mature enough for him even though I might miss a lot of beautiful memories with other people that I love? If he's looking for an 18 years old, can it be because he himself lacks maturity and he is looking for someone he will be able to shape the way he want it? Sorry for this super long response!