I am 54 and employed in the government sector, working in IT. For almost my entire adult life, I've also done consulting work on the side, mostly obtained word of mouth. For the past 12 years I've done a gig for a local company where they pay me a monthly retainer to do side work for them exclusively, that retainer has been more than what I make in my "full time" gig, mainly because I know I am underpaid in my full-time job (I made a conscious decision early on I would be trading salary for more PTO and a defined benefit pension after 20 years.) Unfortunately it's about to come to an end over the next couple of months as the company has been sold.
I have had many, many people over the years ask me why I do not do consulting work full time. To principle reasons:
Inherently I am an introvert. Not that I cannot be an extrovert when I have to be (e.g. I routinely have to give 60-90 minute presentations to large audiences of 100's of people, engaging in Q&A etc), but I am a classic ISTJ on the Briggs-Meyer chart. I married an ENFP to compliment me at cocktail parties. Remember: As stated by another above, a major portion of consulting is marketing yourself, building a network, etc. Always being on the lookout for the next "gig". I find doing things like that to be emotionally tasking and exhausting. Someone else pointed out when you become a consultant, you're really becoming a consultant/sales/marketing/administrator, and this is more true than I can tell you (I pay myself a salary from my consulting work so I can accumulate Social Security quarters, given my government sector job doesn't accumulate them and its either I pay an accountant $200/mo just to pay myself, or do it myself.) So, unless you're an extrovert with great networking skills, consulting probably will not pay off for you (of course you can always hire someone to market you.)
Health care. Moral of this paragraph: expect to spend a significant amount of money on your health insurance, and factor that in accordingly into what you're going to have to make in order to live. Case in point: The health insurance thing here (USA) is a major concern, especially now that I'm getting older and more prone to health-related issues. I have decent health insurance through work (not great, but not bad either) and over the past 10 years I've seen my costs skyrocket. Keep in mind what we usually see for our "insurance deduction" through work is only a portion of what the actual premium is. So, for example, my deduction is $360/mo, and my employer pays 80% of the premium, hence the real cost of my "single" premium is really $1,800/mo, or $22k/year if I had to pay it myself in full. And that's a rate in a HUGE (government) group, and like I said, while the coverage is okay, it isn't one of those "platinum plans" you hear about. [My wife is from Brighton/Hove, and she swears the NHS is the best thing since sliced bread compared to the US system, but, I often have to remind her: "honey, you're only 37 and haven't had any major medical problems -- see how great socialized medicine is when you're seriously ill and have to wait for live saving treatment". I know transplanted Canadians whose family members have died waiting for live-saving treatment.]
Sure, the office politics can suck, and the work can be unrewarding at times, but in my case I've felt those downsides are better than the alternatives.