The unspoken secret is that the single most challenging work you can do in aviation is single pilot IFR. Flying large airplanes, particularly airline-type operations, is far more simple, and far more automated, with the work-load far more divided.
Flying a light airplane in the ice, in the clouds, at lower altitudes, often without radar, with the full workload of doing all the navigation, all the communicating, usually all the flight planning, and every other aspect associated with making a flight go, be, and end safely, is a challenging and busy endeavor. It's a rewarding one, too, because you're connected to the airplane and the flight all the time in a way that one most definitely is not in an operation such as an airline flight or corporate flight.
In a navajo, one has very little performance remaining with loss of a powerplant. In a turbojet airplane, I'm guaranteed performance within known standards; I can lose an engine and execute a missed approach, or maintain altitude, or fly to another destination; that isn't always possible in a piston twin. In fact, when you fly that single engine ILS, there's really no possibility of going around, and one shouldn't try, no matter what anyone tells you; it's a commitment to land.
Flying air taxi and charter can be hard work and a demanding activity. It's not for the faint hearted. It often doesn't pay well. The hours are often long, and many operators tend to skirt the regulations as much as possible to have pilots hanging on the end of a thread around the clock, waiting for a flight. The demands placed on the taxi and charter pilot are generally a lot higher than those placed on the turbojet airline or corporate pilot...this is the little-spoken secret...you work harder, but make a lot less.
Presently I fly a 747, but I've done quite a bit of charter and "air taxi" work in single engine and multi engine airplanes. I find it enjoyable and fun; no doubt you will, too. I find flying in that environment a lot more rewarding, with the flying being much closer to the reason I got into flying in the first place. I love to fly. Hand-flying everywhere, being 100% of the airplane and the environment, is it's own reward. Compared to flying the 747, where much of the trip is spent on autopilot, monitoring systems or working the airplane through the autopilot rather than actually manipulating the controls, flying a light twin is a pleasure. I enjoy the 747, but flying light airplanes is a guilty pleasure that I enjoy more.
33 years old isn't too late to start. It's never too late, but you need to be realistic in your expectations. The money isn't great, the conditions are often poor, and the equipment in many cases isn't top of the line, or maintained particularly well. You don't get to choose your flying times or the conditions, and the real learning, you'll find, didn't happen in your career when you were training...it happens out on the line when you get some real eye-openers.
If that's your dream, chase it. If you approach it realistically, there's no reason why you won't be quite happy when you catch it and achieve your goals. Aviation is an adventure simply because of this; we know where we start, but until we take off, we never really know where the journey will lead. You can only pick up one end of the stick to see what it weighs; when you pick up one end, you pick up the other,and who's to say what that end holds in store for you? Go for it. You'll be sorry if you look back and wish you had.