If we run with the Pew Survey figure (63% attrition) which specifically refers to adults who no longer identify as JW , I would roughly hazard a guess at about 30% - 40% who leave in their late teens and another 20% - 30% who fade / get dfd when they are older.
What the figures don't say is the quality of 2nd / 3rd generation "brought up in the truth" JWs. I find most of them are essentially just "social JWs" - going through the motions simply because it is all they have ever known. Yes they attend most meetings & do a bit of ministry but I don't really see much difference between these ones & the average non JW of the same age. Young JWs are very successful in compartmentalizing their lives - doing what they need to do to keep everyone happy , keep the elders off their back and keep the parents happy that they are "in the truth" while living what is in some cases a more "worldy" lifestyle than the average "worldly" person. A young JW I know works for a business associate - he told me once that "you would never know that she was a JW".
One of my older distant JW relatives has about 8 grandchildren , of whom only 1 has stayed a JW , presently in good standing. But here isn't really any difference between her & her cousins. They all hang around together and mix with other JWs & non-JWs.