Top 50 at age 15.... great achievement.
At age 16, Hingis, Seles and Austin won slams.
Times were different then - just as older players are more competitive than they used to be, so younger players are less competitive. There was, I believe, a full decade in which not one woman aged 18 or younger reached a Slam quarter-final - from Golovin at US Open 2006 to Konjuh at US Open 2016. As others have posted, there are structural barriers to young players that didn't use to be in place - Gauff can play fewer tournaments, so she can get less experience. And the game requires much more physical strength, which is a disadvantage to people whose muscles haven't finished growing. We are really unlikely to see 16 year olds win Slams again for a while. Indeed, the last one was in 1997. Success isn't just about individual talent: players need social conditions to be in their favor. Back in the day, those conditions favored young players and hindered older ones. Now it's the reverse.
BTW, we might also note:
Capriati reached a Slam semi at 14.
Jaeger reached a Slam semi at 15 and a Slam semi at 16 (she played the final the day after she turned 17 but I think the semi was the day before she turned 17).
Graf reached a Slam semi at 16 and won a Slam at 17.
Fernandez reached a Slam semi at 17 and a Slam final at 18.
Sanchez-Vicario reached a Slam quarter at 15 and won a Slam at 17.
Sabatini reached a Slam semi at 15.
In those days, many players didn't improve much after they turned 16. But if you look at Halep, Barty, Pliskova, Stephens, Svitolina, or even Osaka and Andreescu, you'll see that players are still making marked improvements for years after that these days. Halep and Wozniacki were both in their mid-to-late 20s when they won their first Slams. Kerber was 28. Li was, too. Pennetta was 33.
