I can't pinpoint when I became a grown up but I think it has to do with becoming more aware of my surroundings. I remember going shopping with my mom and running to bring her something to buy for me that I just had to have. "How much is it? Did you check the price?", my mom would ask me. Every time she asked I would sigh, run back to where it was and find out the cost. Being clueless about the cost of the things and separating a want from a need is a big part of growing up and it's coming into view now, crystal clear. I still have that little kid inside that absolutely wants and needs to have something right this second but growing up has incorporated some patience. Some discipline. I've never felt it as strongly as I do now.
The biggest thing I have ever planned in advance was our wedding. Thirteen months. Now I find my husband and I planning trips, goals, get togethers, etc WAY in advance. I'm not used to it and it's uncomfortable at times. Just going to the movies with friends is planned at least a minimum of two weeks in advance and we don't even have kids yet. ;)
Life is short but making plans and being patient has its perks too.