Disinterested Third Party
I'm a long time agricultural pilot. Lots of spray time, lots of fire time in everything from cubs to pawnees to Dromaders to AT-802's. I've been doing it since I was a teen.
Let me say first that with your experience level, more than likely your uncle is talking about having you do ground work...loading and mixing, rather than flying. You can't work commercially, although you could spray legally for private use (family). Getting insured, however, assuming that your uncle runs insurance on his gear, will be a real bear.
Spray work is about a lot more than flying the airplane down the field. It's specialized flying, but the monkey skills of piloting the airplane are just a part of it. Ag pilots are usually called on to visit fields, examine crops, make recommendations, handle and mix chemicals, deal with hazmat issues, perform maintenance, and a host of other issues that are a part of ag life.
A 50 hour ag pilot stands a good chance of being a dead pilot before reaching 100 hours. Years ago the national aerial applicators association did a study that put the average working lifespan of an aerial applicator (crop duster) at seven years. That's counting the guys who get killed in their first season, and the ones who spray a lifetime.
Changes in the industry have expanded those numbers in the intervening years, I'm sure, but the truth is that the ag industry still sees a disproportionately high incidence of crashes and fatalities. There's a lot going on in ag work, from steep turns and work close to an accelerated stall at 75' above the ground to navigating powerlines and obstacles in the field.
A single drift claim from spray drift can put an operator out of business or destroy profits for the entire year...one person claiming your spray damaged their crop or trees. Spray work involves using some chemicals that are more toxic than you may have thought possible...a single drop of Parathion 8E taken orally will kill you. Some spray jobs require regular blood tests to monitor certain chemical levels in your body.
Bear in mind that in ag work, a helmet is worn for a reason (bear in mind they cost between 900 and 1500 dollars, too...a little more than whatever headset you may be wearing). They've very necessary. Mine has saved me, before.
You should also be aware that the FAA considers even the logging of flight time to be compensation, so that whether or not you actually get paid becomes a mute point if you're doing commercial work. Even if you're not acting for hire or compensation, simply logging the flight time has been treated by the FAA as compensation when considering violations of the regulation by private pilots.
There's nothing to say a young man can't be an ag pilot. It was my first job out of high school, but my experience was all conventional gear and ag time, and it's what I knew. If you're hopping out of a Cessna 152 and hoping to go fly your uncle's Air Tractor or Thrush or Ag Cat or Callair or Brave or Ag Wagon...it's not simply a matter of hoping out of your rental airplane and into a working airplane. Ag aircraft have been known to routinely eat a pilot's lunch...and that includes some very experienced pilots.
You indicated that you'll be working a combine. If you're hoping to use that as a foil and pretend you're getting paid for the combine work rather than the flying, the FAA isn't that blind. It's a subject that regularly comes up for enforcement. You can get compensated for flying in your job, but only if the flying is incidental to the job. If you're flying yourself and some workmates to a worksite and you were going there anyway by car, train, or pet dinosaur, you're within the realm of reason. Getting hired to operate a combine, however, isn't the same as flying the airplane, and if you're spraying fields in a single seat airplane which has no other purpose than to dispense an economic poison (crop dust), then you're squarely in the realm of doing commercial work without a commercial certificate.
You will also need state certification in each of the areas you'll be doing as an ag aviator (pesticide applicators license, herbicide, etc).
Your uncle, if an ag operator, should be able to explain the basics. Be aware, however, that if you're contemplating a career outside ag, operators tend to think of their own business. Many don't understand what flying outside of ag is like, and consequently don't fully realize the impact that a powerline strike or certificate action could have on your career. Many employers out there (corporate, airlines, etc) tend to view ag work as cowboy work, and it can be seen negatively. I've dealt with that first hand. Never mind that it's precision flying...there are too many stories out there about ag work for the popular misconceptions to die away. You need to understand that your uncle, if a died-in-the-wool ag operator, may not understand aviation outside of ag. If that's the case, be careful about the counsel he may give you. He may be just fine with having you do commercial work on a private pilot certificate. Rest assured the FAA won't be nearly so understanding.