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.
But she wasn't the youngest ever. Read about ten such young doctors. Link -via the Presurfer
I was lucky to get my PhD so early because my experiments worked :)