Also in Finland, its really common to see people still in university in their late 20's, so that will really drive down the "Neet 15-29" figure. This is because people start later (partially due to military conscription) and because most people go for a master's degree rather than stopping at a bachelor's degree. Compare this to the USA where its standard to be done around age 22. It would be nice to see comparisons between countries for NEET 30+ to see if the differences remain.
A couple of additions on why people graduate from uni so late here.
First, it often takes multiple attempts to get into one. The most popular programs have admission rates of around 10%, but since "it defines the rest of your life", why not try at least once? Then the next year, or year after that, if you're still not in, you can try something more realistic.
Second, (my experience, 15 years ago, your mileage may vary), the whole thing is just inefficiently organized. Where I went, there was 4 six week lecture periods per year, each followed by a week or two of exams. Many courses would have mandatory coursework you can only do during the lecture periods. In the end you're left with 28 weeks of crunch time per year, and the remaining 24 weeks to just fill with whatever.
Most choose to work that almost half an year, some lucky ones actually find something from their own field, though the odds go up as you're getting more years in. Then at some point you find the job more satisfying (because it's nice to actually get paid) than completing your MSc, which you then finally scrape together before having children at 30-something (if even then; I know I didn't).
On topic: I think most people on this trajectory wouldn't settle in the NEET column anyway, at least for extended periods of time.
