It's not too late but it is definitely not ideal if you want to retire by, let's say, 60 years old. At 30 years old you are looking at 3-4 years to get undergraduate / prerequisites. Then 4 years of dental school assuming that you get in on the first round of applications. This puts you at 38 with lets say $280,000 in debt (this number can drastically increase depending on what school you attend).
The biggest issue isn't that you are starting dentistry at 38 years old, it is that you are starting dentistry at 38 years old with $280,000 or so in debt. If you have the drive and self control to tackle the loans aggressively once you graduate then you may be able to take care of it in 5-7 years. I have a few friends who have done this and lived like they were still in school when they started making their real money as a dentist. Most of my colleagues however make minimum payments or at least did so the first 5 years out of school and are now taking the loans seriously. However they started working as a dentist at 26/27 years old and had time to catch up when they finally came around.
When you finish 8 years of difficult studies, it is very hard to stay the course with your lifestyle. At this point many dentists feel that they deserve a new house, a new car or to finally go out and eat fancy dinners multiple times a week. All of that will add up and just put you further behind ever retiring if you go that route.
Some don't mind paying the loans until they die as they don't want to give up their lifestyle and just see it as the cost of doing business. Some don't like the subconscious daily reminder of how much debt they are in that affects their daily life due to the constant stress. I personally did not want the stress of debt weighing on me for years. There is going to come a time where you are tired of having to work. Not tired of working but tired of HAVING to work just to pay the bills. Life is not all about money, but having money allows you to focus on other things and to be financially free so that your career no longer controls you as you can walk away at any point. If you are in a ton of debt working as a dentist, you may like doing dentistry - but I guarantee you wont like HAVING to do dentistry to stay afloat. The stress of a simple wrist injury having the ability to derail your entire life because of your debt and monthly needs is stressful. You might convince yourself that you don't mind working into your 60's, but your subconscious always knows that you don't want that. There's a difference between choosing to work 1 or 2 days a week in your 60's because you like dentistry vs HAVING to work 4-5 days a week in your 50's just to make your monthly payments.
Even if you aggressively pay off your loans in, let's say, 6 years - you are now 44 with $0 net worth (better than (-)$280,000 net worth but still a tough starting point for retirement). The most important years of compound growth for retirement are our 20's. The next best decade is our 30's. After that, it becomes very hard to catch up with someone who lets say has a mechanical engineering degree and has been working since they were 22 funding their retirement with $60-$80k salary. The compound growth that they received by funding their 401k for that decade puts them way ahead of most of us initially, especially if you aren't starting until your mid-40's.
I am not trying to be negative, but I am just bringing up very important points. It really depends on your situation (married / dual income / kids / homeowner?) and what your goals are. I would say that financial freedom is a goal that we all strive for whether we overtly admit it or not though. Just be careful with how much debt you go into in this decade of your life because your decision now will be very important for where you are at in terms of retirement/freedom in 20 years when you no longer have the drive/energy/health that you do now.
