Would it be a bad decision to buy a 1999 Chevrolet Tahoe?

The mileage is actually VERY good for a 1999 Tahoe (or ANY car/truck of that year, IMO). Have a mechanic check it out thoroughly first and let him help you make the decision. You didn't mention that asking price...what's the Seller looking to get for it?

I'll know tomorrow what they're asking. I was just wondering if the age should be a big concern... It's kept up very well though so it may be slightly overpriced.

It's just like looking for a "Cougar". They are an older model, some have low , others have high mileage on them. That's why you take them for a test drive. Find out what you can about the past experiences, ridden hard or only taken out for leisurely cruise. Low mileage on the Tahoe might mean it was only used for family vacations or not driven during the high gas prices. Was it a soccer mobile and used to hall kids in town? Any item this old will have wear marks on it, expect them and be prepared for normal repairs. Your not getting a new Mustang or SUV either. If the price is right, and she is sound, take both.