I wouldn't do it with powders over 5 years old, but under that would have no problem with it. Truth be told, 10 or 15 years would likely be fine, but there is a chance one is going bad. I have powder 30 years old that looks and smells fine, but I had the better part of an 8 pound jug of rifle powder go bad somewhere between 10 and 15 years.
Many people put the last little bit of X powder in the new jug of X powder and mix it up. Plinking ammo? Go for it. Match ammo? Not me, unless they are both the same lot. YMMV.
Here is some surplus .308 from 1982. In 2014 I noticed a few rounds showed crud/corrosion around the neck where the bullet meets the brass. I started pulling it apart a few weeks ago. About 15 to 20% of the powder showed deterioration, with about 10% showing green corrosion. Yea, 32 years old, and we don't know how it was stored before I got it, but it doesn't always take that long, especially in heat.