I have been wearing long sleeves for anything out doors for some years now, mostly to save on sunscreen. My Irish ancestry leaves me very vulnerable to sunburn and skin damage especially at the altitudes where i like to be. Recently I have switched to lomg pants as well, same reasons.
Interestingly, as I move into my new fifties, I find I run much cooler than I used to when stting still, while still running as hot on the move as ever. So base layer shirt and light pants are still fine hiking at 40 degrees, but I need more warmth for rests and in camp than I used to, and if I start out cold it takes longer to warm up. Plus I have had thermal injury to my hands (chilblains) , Â as well as some Reynauds Syndrome, and have to be more careful to keep them warm.