I think 2.5 *can* be reasonable, but it very much depends on the kid. Our older one had his "first solo" at around 2.5, after having flown many times previously with mama aboard.
Obviously, you don't take the kid without assistance the first time. You need to know whether they'll like flying or if it freaks them out, whether they get airsick, and how well controlled their emotions are. Flying is a crazy thing, and every kid reacts to it differently.
@Mtns2Skies has flown with his kid plenty, though.
Here's a few things to do or take note of:
1) Figure out seating with an eye towards emergency egress. You want the kid to be able to get out on their own if you are incapacitated, and if they need a car seat it needs to be installed in a seat where it won't impede anyone else's emergency egress.
@Mtns2Skies with the tailwheel Cessnas will have an advantage here with two doors. For a low wing with a single door it gets more interesting, and there's usually only one seat (left rear) where it's safe to install a hard car seat. However, we've had good luck as the kids get older putting them on an Oregon Aero booster with the aircraft seat belt. Unfortunately, it doesn't appear that they sell it any more - The bottom is about 6" thick so it gives them a nice boost, and it does not attach to the airplane at all so it won't impede egress.
2) Behavior. With my older son, we took the time to just sit in the hangar and practice not touching stuff! I let him ask all of the questions he had about the stuff he could see, and let him look and see how the yoke worked and talked about why it was important to not touch it. If it's a kid that frequently doesn't listen, you might not want them in the front seat. In either case, for the first time in the front seat, it's good to have an adult sitting behind them so that they can restrain the kid if necessary.
3) Hearing protection. I'm sure
@Mtns2Skies has this one figured out already, but if you've never flown with the kid before, you'll need something. Our kids have worn adult headsets starting around age 2, but they have big heads.

Em's for Bubs worked up to about 1 year; Em's for Kids from 1-2. After that they want to be able to hear you talk, and be able to talk back to you. Definitely know how to isolate your audio panel for whatever seat(s) you have kids in.
Also, IMO it helps to give them something to do. My older son gets to tune the radios when he flies with me for example. In addition, give them some level of control if you can - Tell them they can decide when to go home or to land or whatever. Keep it fun so they'll want to go again!