Hello Oggagala.
The 18 year olds on the National Team are there because they are on teams that will "go bot" soon. When a team goes bot, all of the players on the team are retired. However, if a player is on the National Team, they become a free agent instead of retiring. Our coaches do a great job of scanning the promising young 18 year old players and then using the roster space on the NT to save those players. Hopefully, some newer American users (people like you!) will buy those players and train them into future U21 and NT stars.
As to your second question, the final roster is not yet set for the U21 team. Additionally, the indicated position for each player is not necessarily the place they will play. More importantly, there is a very limited player pool for the U21 coach to choose from. If no one has trained any real PG's for the U21 team, then the coach has no choice but to use guys listed as SG at PG.
Hope this makes sense...
http://www.youtube.com/watch?v=oHg5SJYRHA0