Personally, I've seen a few pilots that I wonder about as well. But that's not where I'm going to go with my response here.
There are two reasons that I can see for one to have second thoughts about an "old" FA.
The first, as I mentioned before, is safety.
The second is extraordinarily politically incorrect, but I'll mention it here anyway, after first noting that I do not share this opinion.
In a service industry, appearance has a lot to do with the customer's perception of the quality of service. This was what led initially to the "coffee, tea, or me" days of commercial aviation. I suspect some of the FAs on US flights today were once among those PSA FAs with the hot pink hot pants. 🙂 But I digress.
The point is, service industries tend to do better, all else being equal, if they have attractive people facing the customers.
Having said this, all else is generally not equal. Hiring bimbos is not going to work because good looks won't cover for bad service. Furthermore, there are certainly ethical issues with terminating one's employment due to physical factors unrelated to one's ability to perform one's job. It is for this reason, first and foremost, that I object to the practice.