In reply to mheyes101:
If you want to do this then a good way is to join a walking club or meet up group. Here you will get a chance to lead groups of mixed age and abilities walkers (a lot of novices/inexperienced). It is also good practice in planning and preparation, providing walk information as well as practice of being on the hills, dealing with group dynamics, emergencies, weather etc. Also volunteering for youth groups, DofE, cadets is good practice.
One of the best bits of advice I can give is to walk in as many different mountain areas in the UK with as many different types of groups as possible. Being a mountain leader is more than just being able to navigate day and night, being able to provide info on the different mosses, outdoor access etc etc. Being a mountain leader is about how you interact with others and how you make people feel, inspire, motivate and educate them.
You can often spot ML trainees who have only walked solo or with friends/family a mile off, they often walk too fast, their kit is less catered for group walking and sometimes they can lack confidence with certain groups of people, particular those of a more vintage age.
Best of luck and enjoy getting out on the hills!