Good news here. But only one...potentially two...18 year old in the top 100? Wow.
I think he's sure to do it now, as he's #99 in the live rankings for the moment and only Rola has the chance to overtake him (Menendez Maceiras lost his QF and can't). So at the very least he'll debut in the top 100 as #100.
But yeah, I see your broader point! Zverev (the 18-year-old one) is reasonably close at #119 - only needs one good Challenger run to push him over. After him, Donaldson is #166 in live-rankings; then all the way down to Garin #233, Khachanov #236, and 17-year-old Rublev #292. Those are the only 18-year-old-and-under guys in the top 300.
(NB: 17yo Tiafoe, in the SFs in Savannah, could break the top 300 if he wins Savannah. He'd go from #450 to the 290-range in a single week.)
Players under 18 in the top 100 next week: 2. Top 200: 4. Top 300: 7 (perhaps 8 depending on Tiafoe). That's ... not high.
The plus side of this is that it's possible to raise your ranking really quickly if you have the talent (and a smidge of luck). Tiafoe was #1143 in January; since then he's turned 17 and raised his ranking
significantly. Kokkinakis (who turned 19 a week ago) went from #415 a year ago to #104 this week. Kyrgios went from #843 to #171 in a single year, and then from #171 to #35 in another year. So teenagers that have talent can rise very quickly from the bowels of the rankings right up into competitive territory. Besides Tiafoe, I'm keeping my eye on Jasika, Kozlov, and Lee, for example - all ranked from #386-#433, all under 18, and all showing promise.
