I know a couple of doctors who worked until they were 70, and then retired. However, both appeared to be in very good shape at 70 and very much able to work. How much this was motivated by finances versus just liking the work I am not sure.
In terms of retirement planning, to me it is not clear that taking a gap between undergrad and medical school should hurt at all. Presumably the student will be working in those gap years, and could contribute to an IRA during that time.
If a parent can afford to do so, then I think that it is a good idea to help your child contribute to an IRA early, for example if they are taking a gap between a bachelor’s degree and graduate school. Even relatively modest contributions to retirement funds, if made early, can make a big difference 40 or 50 years later when retirement age comes around, or 60 years later when someone has been retired for a while.
I think that this is possible. Our daughter who got a DVM took a gap, and a big reason for the gap was to make sure that she really wanted to take this path. She did know a few (very few, but not zero) students who dropped out part way through the DVM because they decided that they just did not want to do it. Taking a gap and figuring this out before you start to me seems like a far better approach.