This question begs a different answer depending on different people. What is really a comfort zone? It just means at what level of something you would be comfortable with or happy to be content with. I mean when it comes to salary what is the comfort zone in you area. Some people will feel that say 70k to 80k, they will be happy and comfortable to live with and for some the salary can go into the six figures. And even after that some feel like a seven figure salary is their comfort zone. Same thing happens in case of buying cars and houses and other household stuff. Some feel comfortable with a small inexpensive car but some want a very expensive car although both take you to the same destinations.
For some people even if they are in their comfort zone, they feel that they can achieve something substantially more by working towards some other goal they want to achieve ( I am one of them). But overall I have seen that most of the people are just fine with where they are and even if they are not, they are not doing much to improve their positions or pushing themselves to their limit. I believe that with 24 hours in a day you can still achieve more even if they have kids since you just have a small window of time before the age factor drags you down.