Our advice is don't appeal them. We find they don't last forever - unless you appeal against them. Here's a screen shot of the analytics from one of our films.
You can see the times it has been age-restricted (almost no views) and the times that restriction has been lifted. In its life so far, it has been age-restricted three times and has achieved nearly 5m views.
I have asked YouTube about this. Tey have no idea how or why the restriction is lifted - but, in the films where we appeal, the restrictions are not lifted.
What about doing the age-restriction yourself? in the short term, it slows down the chance of that video getting a big audience. Age restriction means the video has no chance of appearing in the crucial Suggested Videos column on the right of the YouTube screen. However, it is not disastrous.
Here is the graph for a video we age-restricted ourselves.
Where it has done well is where someone on another social media platform - usually Twitter - has shared the video. Age restriction is chic! This film has still achieved 1.25m views.