My mistake. I didn't read the entire article and misread this line, " A monetary union was established in 1999 and came into full force in 2002, and is composed of 19 EU member states which use the euro currency," to mean it was founded in 1999. Reading further down, I see that it was founded in 1993. Still, we're talking about an additional 6 years.
But the EEC, which pre-dated the EU, was formed in 1958 and thus is considerably older. Being a filthy American, I'm not super clear on how much that distinction matters but I do hear a lot of talk on the news implying or outright asserting that the age of the "EU" is quite a bit older than ~20 years, so it seems like in some circles, as least, the time from the treaty of Maastricht is something of a technicality w.r.t to the age of the institution?
Right, and my understanding is that the UK is voting to leave agreements going back to that (considerably) earlier time as well. If so, even referring to the 1993 date is really missing the point, as the implication that the UK is leaving something that's "only" been around for 20ish years isn't really correct.