For under-23 GT top-10 GC performances in recent years, we have:
Pogacar, 2019 Vuelta, 20 years, 11 months: ??
Bernal, 2019 Tour, 22 years, 6 months: 1st (also 2018 Tour, 21 years, 6 months: 15th)
Sivakov, 2019 Giro, 21 years, 11 months: 9th
Barguil, 2014 Vuelta, 22 years, 11 months: 8th
Pinot, 2012 Tour, 22 years, 2 months: 10th
Gesink, 2008 Vuelta, 22 years, 4 months: 7th
Schleck, 2007 Giro, 21 years, 11 months: 2nd
Am I missing anyone?
Only Bernal's and Schleck's performances would be comparable to a potential podium finish from Pogacar, and in both cases they were at least a year older.