Why do I still feel like I'm a teenager??

I (29F) feel reeeaaaallly behind on some things. I see my friends go to grad school, get married, have babies, etc., but I just cannot bring myself to envision such a life for me. The problem is I want such a life soooooo deeply but feel like my emotional, financial, and physical capacity to do so is not mature enough to handle any of the real adult things. I still feel like I am mentally 16, make decisions that none of my other friends would even consider, and have a grip on my emotions like holding on to sand.

How does one get to a maturity level high enough to handle all of that without losing the best parts of them? How does one gauge maturity levels without blowing things out of proportion?