I don't know if I am going to properly answer your questions because I am a convinced work-on-your-own person :) but I am going to tell my personal experiences and impressions on that wide subject...
I have been freelancing for over than 20 years now, and during these years happened that I was offered a few times to enter a company as employee and therefore close my own business.
Even though the employee position seemed more secure and comfortable for many reasons, each time I did not accept and I opted to keep the freelance-client work relationship, and in the end I was right to keep it because the secure position with years got weak and (some of) the companies was forced to fire many employees due to world or sector crisis, but I still kept and keep working nowadays with the same companies, because they pay me per job when they need it, and I am not a big fixed cost for them no matter how the businesses goes.
Also for me is not comfortable at all to re-enter to work in team inside the companies, for many reasons:
- First of all I'm used to work alone in my office, when I go to client's offices for a day or so I don't know how can they work with all the noise, talks and distractions. I am more productive if I can concentrate 100% on the task so I ask if I can stay in a separate small room (if possible and according to the work).
- Second when I work there I see that I go a LOT faster and I produce more in terms of quality and quantity than the employees, so that many times I find myself to wait them (sometimes happened - not wanting it - that I put them in bad light with the boss). After 20 years or self-discipline in the work place, where you must produce good and fast results, you naturally go above the standard and that can be a problem for others and for yourself too.
- Other many reasons...
About the age factor, it is very true that after a certain age is very difficult to find an employee position inside a company because they propend to get young people (also for taxes discounts), so if you have this chance think very well if catch it or keep going on as freelance.
I think about that too a lot, what would be like in 10-20 years from now? I am on my 40's and I am completely aware that I can't keep intense work rithms as when I was 20 or 30, also many young people are showing in the business with their freshness and enthusiasm, but I don't worry too much now because I have experice by my side and people trust me as a reliable person and for now I have no problem getting and keeping clients at me (for what are my possibilities as a single person).
If you want to keep one foot in the freelance world and worry for the future or lose passion, I can suggest you to differentiate your business and knowledges, don't just stick to one or two competences but become familiar with more things, that will be also more interesting to move in new areas. Also because a table stay stable (sorry for the rhyme) the more legs it has. If a table has 4 or more legs, when you cut one leg, it still stays up in some way, this is also how many should be your streams of earnings.
For example with time I developed many skills and I work in different areas for very different clients (BTW I always wondered if this is a good or bad thing, but that kept me always with works to do and incomes), I can be a pure programmer, a pure designer, a pure photographer, a full-stack developer, or all of these togheter, which often happens (I'm also certified in some of these areas), so I always find something to do.
But as I said before, I'm one person and I'm not 20 anymore, and sometimes I feel burnt-out of stress and tasks to do, so since some time I am trying to have a side business with my own commercial company applying my knowledges for myself this time and not for others. I keep freelancing with good clients and and tring to grow other areas of business hoping that with years going on the freelance part of incomes will be less and less and the commercial part will become the main income.
Maybe you can try to do a part time employee with this company (for example 2-3 full days a week) and see how it goes and if you find yourself comfortable or not, at the same time you can keep your good clients and work for them in the rest of the time. Then you will decide what is best for you, possibly it will be a very good decision and a very good place to work for many years to come, and I really hope so for you if that is what you wish and decide to do. All the best!