Well, I started when I was 13 years old, and I struggled with just about everything involved with writing. After two years of doing the text-based role plays, I drastically improved with spelling, grammar and vocabulary. This of course helped in my English grades, haha.
It involves a great deal of dedication in learning how to communicate a "Character" with only words. You must animate them, and give them a heart and a soul. Since it involves playing with other people, it gives you a chance to study others writing techniques, and how to improvise and use team work to create a story with many people.
It depends also on where you go to do the role play, what genre, and what story line is given. There are two places I currently do my role playing at:
http://www.willowrpgs.net/ which is a very relaxed setting, and brings together writers of all ages and strengths. This gives you a fantasy realm backdrop, and the freedom to create a character with powers and mythological traits.
http://www.arcticlights-sim.com/ This site is a mixed place, incorporating both text based Role Play, and SIM (Simulation.) It is not currently open to the public for free, though you can purchase an account for very cheap. The RP (Role Play) portion of this site is based in a fictional world, but it nearly mirrors our own Earth. . It is a much more realistic setting, older time period. Also you must be 16+ to participate, otherwise you need parental permission first to join.