I'm a young step mum with an older partner. He has 3 dds. I'm 24 weeks pregnant with our son, my first.
One of them lives with us and I've ended up taking on a mother type role, so I wouldn't say your age is a barrier to being a step mum. Your personality, goals, current wants and lifestyle are all more important factors in my opinion.
When I first came into their lives I was genuinely just one of Daddies friends, so they had met me a few times before we got together. He had them all eow then and I usually spent one day of the weekend with them and eventually this became the whole weekend shortly before we moved in together. Dsd 3 moved in shortly afterwards and we took on a normal family set up. All 5 of us get on brilliantly, the kids have all been 100% accepting and I feel like I'm just as much a member of the family as anyone else. It isn't always easy, sometimes I just want alone time with dp and just can't have it with dsd living with us, but we work around it and it's worth it because to me I'm not just with dp, I've joined a family and I want to be part of the kids lives as much as I want him. The kids aren't always easy, what kids are, but the good massively out ways the bad.
Initially there were times when I felt a teeny bit jealous but it went away. My dp doesn't push me out and neither do the kids which makes a massive difference, if your dp and dsd end up leaving you like a gooseberry then it would be normal to feel like you do and maybe have a chat with him about it. I'd think about cutting down the time you see him when she's there to half a day once a week or one day every other week and ask dp if you can try and spend that day as quality time for the 3 of you, where you are 100% Involved. If you enjoy that then keep at it for now and see how things go, if it's too much for you then hang back a bit and maybe just see her for a little bit once a fortnight.
Whatever you do, do not try and carry on the relationship pretending she just isn't there, that would not be beneficial if you want a long term relationship, she is forever part of him. His parenting style also makes a difference, if you disagree with it you will spend the whole relationship getting annoyed and if you were to say something he may not want to listen.
Now I am able to have this family life because they're all so easy going and including, and because I've always been the family/ maternal type. I don't feel like I'm giving up my life for them, I feel like I'm gaining something in my life. I'm not fussed about travelling or going on nights out with dp etc. If you still want to be doing those things with your dp then this relationship may not work, you need to consider whether or not it is right for you at this point in your life. Some people just want a nuclear family with only kids of their own with 1 partner, if you feel you would be sacrificing that want and that you'd regret that then don't get in too deep.
Also don't forget that how you feel may change, you may feel like you can't handle it now but in a year when you've fallen deeper in love with him and gotten to know your dsd better, you may feel like actually this is perfect for you, you just had to get used to it and rearrange priorities.
As I said I'm pregnant with my first and it has at times bothered me that it's not his first too because he's a different kind of excited. But then it's his first boy and his experience will be helpful for me if I struggle with anything. He is also happier to go with some of my preferences for things because he knows he's had the opportunity to choose before which prevents arguments. I'm so excited to add to our family :)
You need to remember that this is not going to be 100% easy, even if you and your dsd have the best possible relationship and everything between the 3 of you ends up amazing, there will always be an ex in your life who may or may not make things hard (the biggest problem for many of us), and you have to decide if your dp is amazing enough to be worth it.
You also need to think about whether or not your be happier with someone you could be 'free' with.
I think you could either be really happy In the long term or you could end up very unhappy. Just don't give up until you've really considered the options.