Where did you get that idea? I didn't start my mission until I was 20. I am not sure what the upper limit is for a young man, but I think that it is in the neighborhood of 23-25. Beyond this age the young adults need to focus on completing their educations, getting into careers, and/or starting and supporting families. Then later in life, after these jobs are mostly accomplished, members may also serve missions. These "older" missionaries may serve a number of different types of mission, not just proselyting missions.

I was a convert to the church at the age of 20. I was 22 when I finally decided that I wanted to share the joyus news about the restored Gospel with those who had not had the chance to. One of the Elders who taught me was 25. I do believe the upper limit on a single male member serving a mission is 25 for many of the factors that Brother Blaylock mentioned in his answer.

The age range for male missionaries is 19 to 26 years old. Without being too redundant, the reasons for this age range are exactly the reasons Glenn Blaylock said. I also want to mention that many people have critized the Church for not sending out much older people on missions as they are used to seeing other churches have their representatives in their 30's, 40's, and 50's. People see a 19 to 26 year old and they think they are dumb and immature. What is amazing about this, is that having someone this young teaching the Gospel actually helps the message reach the hearts of many. Another doctor I work with said that if she had had a 40 something man telling about Mormonism, she would have gotten really technical and philosophical with him, but talking to 19 year olds humbled her as she was amazed by what they knew. It humbled her heart and she was able to feel the Spirit. They other reason is that this is the perfect age to teach these young men how to plan, study, use your time wisely and effectively. It is the time to mature and prepare for the rest of your life. My mission catapulted me ahead 15 years. I had a couple of friends my age (23) when I went on my mission. They are members but chose not to go on their missions. They are amazing guys, but when I got back from my mission I honestly was ahead of them in so many areas. The only thing they had on me was that they were two years further into their careers than I was. But even with that, the things I learned on my mission prepared me for my career life and I quickly caught up and passed them. I am not saying I am better than them, I am just saying that a mission at that age range prepares you at the most needed preparation time in your life. However, any older than that can become counter productive for the reasons Glenn stated. We are to start families and provide for them. We can't do that as single 35 year old males. So, there is a needed balance of both worlds.