"18 6'1 and 160lbs"
Yep I'd say old and big enough to make it move but eat a steak would ya? You could likely use an extra 10 lbs imo, Mr. Glavine is right, it's finger and seam manipulation, "feel". Rtusk is right also you don't want to stay on the same plane if you can get away from it. A cutter has it's place though, start it outside third and let it fall out of the zone and you'll get some folks casting for ground outs to short. Pronating is, in conjunction with finger/seam manipulation, the best way to work a sinker for movement and action imo, Zambrano and Maddux do that and I don't think you'll find better major league sinkers than those two (With respect to the Webb and a couple of others).
One thing I notice about our sinker/cutter discussions is that those who ask are generally older guys (!6-21) and think that is because of what xv implied, these are finer point pitches, not really something you'll see 12, 13, 14 yr olds working on typically. My son and I started talking these pitches when he was 16 and able to throw the fb for a strike at will...I've found this to be about the average time and age.
RTusk! 8) good to see you posting...How was your season? My son played a team out of Southern Indiana proported to be the best you guys had to offer for 18u at the National Wood bat championships in Atlanta a couple of months back (Beat em 4-2 in a cg gem...had their coach trying to recruit him to what he termed NAIA national champs Southern Indiana not sure what div??..I wasn't there, so I'm asking do you know anything about them?)