Age regression is basically just “reflecting on and reinterpreting a previous experience”. As it’s usually talked about in “hypnosis” circles, there’s usually a bit more brought along with it, and reasons for that.
One of those things is “finding the first experience”. Say someone has gotten bitten by dogs a few times and is now afraid of dogs. In order to help them get over their fear of dogs (perhaps by helping them discriminate between dangerous dogs and safe dogs, or by pointing out that the bites were only play bites and never harmful), you have to access the fear itself. In theory, you could go about it in the abstract and ask “what might happen if you were to not fear dogs?” and they might bring up an emotionally relevant image of a dog bite or whatever. However, people are not always great at bringing up vivid depictions of their actual motivations. They might rationalize, or stay too abstract or any number of other things for any number of reasons. Getting to a concrete experience is a good way of homing in on the actual emotions driving things, so “let’s look at what actually happened to form this belief” isn’t a bad place to start.
In addition, the first instance is often (though not always) the most memorable and emotionally impactful. The first time we learn something it may come as a shock, and that stands out. Subsequent times just becomes “yeah, we knew that already”, and therefore convey little *additional* meaning. The first experience then becomes the prototype that we generalize from, and so hypnotists want to address that directly. Parts of the process might be a bit of a charade, but including seemingly irrelevant detail can help prime people to remember more vividly, and help get to a real emotion.
The concept of “regressing” back to the past and reflecting on a previous experience is therefore useful any time you may not have learned all of the right lessons already. You can’t learn everything correctly on the fly, so reflection is hugely important. Without it you can’t really become a fully functional person. You’d get a first impression, and then you’d be stuck with it for the rest of your life impervious to evidence to the contrary. The more hypnotist specific form where you “regress” to childhood or something is going to be the relevant thing to do whenever you have a dysfunctional pattern of behavior that you learned long ago. Insecurities, for example, are often old old patterns of behavior that still drive adult behavior long past when it was learned.
So for something like “quitting smoking”, it’s unlikely that you picked it up as a young child, but maybe you picked it up last year at 35 so it would be technically but non-centrally an example of age regression to go back and say “why *did* I think that was worth doing anyway?”, and that might be a useful question to ask if you’re still trying to make sense of your behavior so that you can quit without this inner conflict. On the other hand, you might just be able to say “I started for good reasons then and I’m quitting for good reasons now” without ever needing to revisit things. Things like “maintaining a healthy marriage” might require learning to tone down your bits of personality disorder, and these things are much less “trivial and cleanly separable decisions that I could easily make differently” and more “embodied heuristics first learned at a young age and then continually reinforced or at least not conditioned against”, which makes finding the generating set of experiences a much more relevant exercize. Questions like “why am I driven to narcissistic tendencies *now*?” might be something you can answer in an emotionally compelling way, but if not thinking back to “man, even as a kid I had these tendencies. I always felt like ____” might be a useful way to get in touch with the things that drive you and help cleave off all the irrelevant stories you’ve told yourself since.
Most of the time positive experiences aren’t the ones we need to revisit (since by definition we’re happy with the result), so that makes it a relatively safe way to play with the skills of revivifying old experiences and having fun with it. If that’s a skill you want to get better at, I’d say go for it. No need to make a big formality of it though