Every 21 year old is different.
At 21 I was mature, I am still the same fundamental person, but as I studied and travelled a lot after this time I have developed a lot in the sense that the world I put around myself, the things I want to pursue and be part of, have all continued to shape me. So for me, anyone I would choose at 21 (and I had a lovely boyfriend the same age as me) would not necessarily stay a good fit for me as time went on.
In fact that is exactly what happened. I had more places to go, more things to do, more finding out who I wanted to be and developing that person. It doesn't matter the age of a partner, I inevitably was going to learn, develop, do new things and develop as a person. That was me at 21, mature, but still likely to move to another country, take up new studies, move with my career, find new passions, meet new friends, explore new worlds.
Other 21 year olds however may have bought a home, or be running a home, have a child, figured out how to pay the bills. It's a different kind of mature. They can be more centred in where their life is, more settled in the direction they are going in, better at "adulting" and happy with that path.
So i think it's really a question of, are you both in the same place in terms of the path you are on and would like to continue on.
The age, means nothing! Not creepy at all. I wouldn't be concerned about my 21 dating a 34 year old. I might be more concerned about her dating a lot of 21 years olds to be honest! I don't think using the word "creepy" is nice at all and I think you should ignore this.
Age differences seem more dramatic when people are younger, but it's about who you are together.
The children are a big part of this it's true. To make that work you will have to be very thoughtful about the children, the childrens' mother who should be on board and part of the team if at all possible and about your girlfriend. Everyone needs to be respected and have their needs met. It is doable but I think a lot of that will come down to your girlfriend and her general feeling towards children of that age becoming part of her life and her ability to manage the balance needed. That is where I would maybe be more concerned as even a 21 year settled in their own home, paying the bills, continuing with their job, adulating along, may find suddenly jumping to thinking about how to manage two kids of that age difficult.
As someone else said, lifestyle.