Perhaps your parents keep you in short trousers because they do not believe that even at 19 (which you must be by now) you are not mature enough yet to be allowed more adult clothes. Age is not necessarily an indication of maturity and a boy of 18 or 19 can still be very childish and should therefore be dressed accordingly. Also, even though you are now old enough to be legally an adult you must continue to obey your parents if still living at home.
I was brought up in the in the 1960s, 1970s, and 1980s and was kept dressed in short trousers by my parents until I was 21. I wore the standard grey school shorts as part of my school uniform at Preparatory school until I was almost 14. I was then due to go to the upper school and into long trousers but because I had learning difficulties and had failed the Common Entrance my parents sent to me to a small special school for backward boys where the Headmaster was willing to allow my parents to keep me in short trousers as they were strongly opposed to my wearing longs.
I remained in grey shorts at all times until I left the school at 19 and then spent a further two year in home tuition continuing to wear my school uniform and short trousers for lessons and at all other times. Being small I looked younger than my age so I could still appear in public in school uniform although in the last year or so I occasionally received a few looks.
The boy sounds as if he needs very strict discipline in his regime. At 18 he most certainly not be questioning his parents' decisions on short trousers.
He needs plenty of good old fashioned bare bottom spanking, corner time punishment lines and early bedtimes. At 18 he should do precisely as ordered by his elders and betters.
If he was my boy the hem of his shorts would become O so short!
@Timothy My brothers and one sister are quite prim and proper, but I am quite rebellious. I am aged 72 years but small in stature ( 5'6"). Though I dress in adult attire (depending on season or weather conditions) in their company I have to secretly keep it from them that I like to wear short trousers with braces over my pullover in the privacy of my home. I have one friend who knows about my secret double life and he comes to see me in my school short trousers and braces, then he wanks me with one hand and the other on my bare thigh. I want him to slap my thighs and make me cry..
@JustWilliam Im 66 and like to dress in grey 'school' shorts, white shirt and tie, knee socks and braces. I also like having my legs slapped. I'm married so have to do this when i know my wife is going to be out for some hours.
