Moms don't stop being moms, but like everyone else, they're subject to the kinds of life changes that can affect both parents and kids when kids reach milestones like turning 18 and leaving for college. No point bringing on grief if that's what your parents are likely to give, and if they're affirmed antis. Smoking your pipe on the DL may be the way to go until you're 100% independent, but at 18, no reason a guy should hide the fact that he's made an adult decision to take up a perfectly respectable pastime like pipe smoking.
When I learned to smoke a pipe in my teens, I enjoyed it from the start, except for hiding it from my mom. I hated sneaking around, but I knew she couldn't possibly approve b/c I was so young. Senior year of high school, when a friend of mine turned 18, he just started smoking his pipe and his cigars openly. My mom took notice and had to ask if I ever smoked w/him. Her question wasn't accusatory, and the way she talked about my friend was actually kind of complimentary. I didn't come out and confess right then, but her comment made me less nervous, and I kept on smoking (feeling a little less guilty). That summer before college, I ended up not going public b/c I just wimped out. At college, though, I smoked my pipe in public and relaxed - other guys my age were doing it, and I fit in just fine.
While I was at home on break, the pipe thing just kinda came up thanks to my younger brother. My mom wasn't even a little upset. "You smoke a pipe now? Well, as long as you stick w/a pipe and enjoy it, I wouldn't try to stop you. A pipe actually suits you." And that was about the extent of getting approval.