torn in between 2

I'm.giing to try to make this as short as possible. I've been with my SO for 2 1/2 years. I have no children but he has 2 that's with us full time so I consider them mines. Me and my SO have a 10 year age difference so we've decided if I don't conceive before he is 35 that's it. I haven't really thought to much about it. he is 32 right now but my step kids have been hinting they want a sibling along with him as well. I'm young and help take care of them but after a MC 3 years ago in a past relationship I haven't thought about making a child. I have a very active role in their life's so his response to that was " you already do it , you'll be a great mom why not ?". financially we are good , but am I emotionally ready ? help !!!!