Normally, the lifespan of a forced air furnace is determined by the lifespan of the heat exchanger, and that is typically from 15 to 20 years. But, just like anything else, some last longer than others, and your furnace COULD last as long as 30 years and if you're way out on the curve, even longer than that.
I would pay a little to have a heating contractor come down and inspect the heat exchanger. If it's still OK, then there's no need to replace the furnace now. But, if the heat exchanger already has a crack in it, then you're already in need of a new furnace and would be well advised to replace the furnace now.
By contrast, a 20 year old hot water heating system boiler would still be considered a "fairly new" boiler. I replaced the boiler in my building when it was only 46 years old, and it wasn't nearly at the end of it's life. I replaced it only because Manitoba Hydro was offering a huge rebate on upgrading to high efficiency boilers and furnaces that amounted to 30 percent of the cost of replacing the boiler, and there were some repairs needed on that old boiler that could have been avoided by replacing instead of repairing.
You might consider investing in a "hybrid" heating system that provides the climate controllability of a forced air heating system along with the longetivity of a hot water heating system. Such "hybrid" systems have a hot water boiler that provides hot water to a coil in the air supply ducting just like the central air conditioner provides cold freon to a coil in the air supply ducting. You need to provide a much more powerful blower because of the additional drag on the air resulting from having two coils instead of one (both a heating coil as well as an air conditioning coil) but the result is a heating system that only needs to be replaced once every 100 years or so.
It's, in fact, COMMON to see 75 and 80 year old hot water heating system boilers still operating in old buildings, and I'm kinda sorry I made the decision to replace my old boiler when I did. I paid a bunch of money for two new high efficiency boilers, and I haven't realized that much of a savings in fuel costs.
I was promised big energy savings that would supposedly pay off the initial cost on my new boilers within a few years. Truth is, there has been savings, but not nearly as much as I was led to believe I would realize. So, don't replace your heating system because of cost savings in fuel as you're likely to be disappointed. Replace the heating system when it's needed, and at that time consider the option of going to a hybrid system to get your heat from a hot water coil instead of a heat exchanger. If I was planning to keep my house for another 80 years, that's what I'd do.