Your age is not the thing you should be worrying about. You only have one life and you should be working to live it as you find comfortable, pleasurable, useful, ...
The particular field you think you want to enter now should also be a secondary concern in terms of its "hotness". Whether it actually interests you and you feel it is worth pursuing is a much more important consideration. Your chosen field, whatever it is, will probably have a different "hotness index" when you finish than when you start.
You will be older in a few years whether you do the doctorate or not. If you don't, then where will you be in your life's plan. My former spouse finished a doctorate at about 40 and went on to a nice career. And, of course, a decision made now can be altered somewhere along the line as things, including your own needs, change.
Whether you need to spend time in a post-doc depends on a lot of things. Many people do now-a-days, but a lot of that is just the job market. If it takes you a few years to earn the doctorate then the market is highly likely to be different when you finish than when you start. Unfortunately you can't say whether it will be better or worse then, but that would be true if you were 20, now.
And being on the tenure track in the US (an assistant professor) is really being a professor. You just don't have quite the same job security as a tenured person, but the job is really just the same: Teaching, Research, Service in some proportion depending on the institution.
So, my best advice is to do what most appeals to your sense of self-worth.