United Kingdom

It's applied only for elections to the Scottish Parliament and local authorities. It began with the 2014 referendum.

Scotland has also varied the franchise, some of which changes are also under consideration in the forthcoming UK bill. One of the more significant is that people who are permanently legally resident can vote even if they don't have citizenship. I haven't read enough to know whether that's something the UK government are considering, but I could foresee lots of rabble-rousing from Farage et al. if so.

Other changes proposed include measures to encourage voters to participate.

In Scotland, I think the changes have only had positive effects. It's been heartening to see how engaged and informed some young people have been, with support from the school curriculum. One aim was to increase political engagement among younger people - initial signs are promising, but it's too early to tell (even after a decade!) whether it's having lasting effects.

The rest of the article linked in the OP points out that including 16-17-year-olds would only add a small percentage to the electorate.