I am in that age group. I need my glasses - and I need things to be bigger. Everything in the Designer looks a bit too small. Especially in the style panel, that I use 90% of the time.
Much like Photoshop, allowing users to decide which zoom factor to have in the Designer interface would be very helpful. That way we can still produce our designs at the browser's standard zoom setting.
You could add this function in a button right next to the Preview Toggle.