I don't think an age GAP is any more of a problem than any other factor in relationships. Mono or poly.
I'm 28 and my girlfriend is ten years older, at 38. 90% of my partners have been ten years older than me. My longest relationship (5 years) was with a man who is 13 years older than me.
Even though I do not think an age *gap* is a *problem*, yes, of course it can cause some issues. God, I was 26 when I met my girlfriend and in two years, I've already changed drastically. The age gap was never a relationship-deal-breaker problem.... but of course, it was noticeable sometimes. It can work for the better and for us, it does. As long as you all acknowledge the age gap and cut each other some slack, I don't see why it should be a giant issue.
However, AGE itself can be an issue, in the longterm. When I was 19, I met a man who was 32. That was my 5 year long relationship. I thought he was my soulmate. I moved across the country to be with him. I thought I'd be with him for the rest of my life. Yes - life is short. Seize opportunities for yourself. But understand and accept that life probably will change your relationships, your personality, your life. You may not be with this couple in five years' time. If you can strike a balance of living in the moment, whilst having back-up plans for the future, you're doing ok.
I'm not going to trash talk your age - I'm just going to give you my honest opinion. I know, from personal experience, that having one child is difficult. Three would be insane for me. I also know, from personal experience, that I would not want to bring a child into the world until I had my shit together. I cannot tell you that you are too young - your body and mind are yours. But, if you cannot look five years into the future and say "here's what I'll do if we are no longer together", then you've got to do some thinking. If you can, then fine. But be sensible... explore yourself... live your life... love and be happy. I can tell you that when I was 18, I felt very mature - and I was mature. Much moreso than other people my age. I couldn't imagine being much different when I was 28. Hahaha... I was so wrong. I had a massive change between 18 and 21. I changed again around the age of 25. Now, at 28, I'm starting to change again. These are standard ages to go through changes... so as long as you are prepared for that, you are being wise.
me: female, 29
GF: my primary girlfriend, 39
3 year, open poly V, long distance