It's quite impressive to know that Alex Santoso received his PhD at only 25. But it's not a world record. There are outliers, childhood prodigies, who skipped grades left and right to get a doctorate at unbelievably young ages, like Ruth Lawrence.
In 1985, at the tender age of 13, Ruth Lawrence graduated from Oxford University with a bachelor’s degree in Mathematics. The next year, she got a second degree, this time in Physics, which was followed by a PhD in Mathematics in 1989 when she was just 17. After spending some time at Harvard as a junior fellow and working at the University of Michigan as an associate professor, Lawrence moved to Israel. There, she became an associate professor at the Hebrew University of Jerusalem.