There are so many different factors that it would be difficult to go into here--it would be a very good conversation to have with your OB/GYN.
For instance, I have Poly-Cystic Ovarian Syndrome (PCOS) and endometriosis. Either one is a fertility inhibitor alone, but together it makes it VERY difficult to get pregnant. PCOS can be determined during an ultrasound. Endometriosis is usually only found during surgery, though you can be diagnosed just with symptoms.
And honey--I'm 38. I had my son after TEN YEARS of trying. I would have liked to have had a child by the time I was 26 as well, but it doesn't always work that way. If you have tried consistently to have a child and have not conceived, it is time to see a doctor for more specialized advice for YOU, with a full checkup and discussion of your medical background.