I also had a similar path where I first started noticing a very mild stutter around age 12 and it got significantly worse overtime, especially around the middle of highschool. It started off as annoying but very, very mild repetitions or the occasional block but nowadays I have a moderate stutter with frequent blocks. I think it probably had something to do with puberty and brain changes/hormones. I heard during and after puberty the brain becomes more sensitive to dopamine. Its hard to say for sure what causes this sudden appearance in hard blocking and I guess we will never know which is what makes this soo frustrating. I think my father also had some stuttering issues when he was younger so perhaps its just a gene that activates during puberty.