My 19 yr old son is between a Large and an XL in most t-shirts.  These are a bit small for him but his size always varies with different brands.  I can still keep and use them for my 10 year old.  He can wear them to bed for now or to play in the house on weekends.  When he grows closer to the right size they will be alright for him to wear to school, as long as he doesn't get them too stained.
The shirts are very soft and wash well, they didn't shrink even dried on high heat.  Love this brand!!