There are 7 years between my dh and I. When we met, I was 28, he was 21. The age gap, though admittedly not as large, has never really been an issue. We had very similar life goals/plans and similar values. He was also quite mature for 21 (certainly more mature than the men in their 30s and 40s I'd been dating up to that point!). When we go married, he was 24 and our first dc was born when he was 25.
But two things, I think there is a big difference between 18 and 21. Those years are pretty formative. He'll either be just starting uni or just moving out of home and entering work. Living independently for the first time, making new friends, figuring out what he wants out of life. When I met my 21, he had a final year left of uni, a career path, a stable group of adult friends, had lived independently for 3/4 years, had travelled and also worked abroad in a developing country (we met while both working in said country actually). He was sort of ready for the next stage of life, settling down, etc. I'm not sure most 18 year olds would be at that point. Neither of us had any children at that time either, which is a huge complicating factor.
That said, there is no harm enjoying his company. I would just keep your kids out of it for now and anticipate it may not move beyond being a casual thing. If it does become serious in time, great, no harm done either.