But quantity, sure. And that's kind of what the article gets at. Universal and free public school for example isn't all that old, and while it's fortunately present almost everywhere on the planet today, the average length of studying differs a lot.
Kids in low-income countries current go to school between the ages of about 5 to 11, and then quit. Whereas in OECD countries, kids are generally obligated by law to stay in school until age 18 or so (used to be 16-17 but it's shifting up a bit nowadays).
That's a 7 or so year gap of teenage education that's completely missing for the vast majority of kids in low-income countries.
The 100 year gap in the article concerns the notion that in the year 2100, kids in low-income countries will have bridged this gap. By then, even today's low-income country kids will get more than 10 years of education, which developed countries get today, bridging the gap.
Graph from the article explaining what they mean by the gap:
http://ichef.bbci.co.uk/news/624/media/images/82466000/png/_...