Psh, 19 isn't old! Lacey is 25 and I still trailer her all over (she's spent atleast 10 hours in the trailer since this time last year alone). We also go on 4 hour off-property trail rides on a regular basis (lately it's been about once a month) and she loves it. She's still raring to go when it's time to head back to the trailer.
As long as he reacts well to the stress and is reasonably fit, I don't see a reason why that would be a problem. :)
Fabio - 13 year old Arabian/Lipizzan gelding
Rest peacefully, Lacey.