Special interests last *how* long?

This Lifehacker article:https://offspring.lifehacker.com/why-you-should-encourage-your-kids-very-specific-obsess-1821246469suggests special interests last 6 months to 3 years?? Hmm. At the Engine Shed – South London, we know teenagers and adults who still enjoy building and playing with Brio and Tomy train sets; I know teenage autistic