How common is it for people to realize they have Tourette syndrome as a young adult, and how does that prevalence compare to onset in teenage years and onest in childhood?

Not as common as you would think. You need to understand that Tourette's has a range of ways that it manifests itself in people - from severely visible and audible tics to the barely recognizable. Also, most people, unless they have it or know someone who does, do not have any idea about what Tourette's really is. Unless they are diagnosed as children, most adults will never really be exposed to any information or feedback from anyone credible, that would help them understand what they have. Also making it difficult for adults, is the stigma attached to Tourette's due to the jokes in countless TV shows, Movies, and more. The way that it is painted to everyone makes it seem like Tourette's is one thing  - and when they compare their own tics to what they see on TV, they dismiss what they have as simply a nervous tic. They certainly don't want to be placed in a category with what is made fun of. The truth is, I know many adults that seem to clearly have Tourette's, but when asked about it, deny it, scoff at it, or ignore it. There is no one assigned at your job to help look for disabilities, syndromes, etc. There is no counselor or nurse or teacher to help identify what parents miss. Adults are on their own after a point, and Tourette's is still far too misunderstood and unknown for most adults to ever self-diagnose or get diagnosed by someone reputable.