Twin beds are usually the best size for 13-year-olds. However, it is important to note that as adolescents grow, a new mattress may be necessary to support their development. Also, comfort is another essential factor in selecting an optimal size for a pre-teen or teen.Twin beds are usually the best size for 13-year-olds. However, it is important to note that as adolescents grow, a new mattress may be necessary to support their development. Also, comfort is another essential factor in selecting an optimal size for a pre-teen or teen.