The age of a gun might be the reason it does not shoot well. Older machinery and materials, different tolerances or changes in ammunition standards all can play a role, but since there is no easy way to remediate any of this, we will leave the seasoned old timer alone.
Modern advances in manufacturing helped increase standards and raise expectations. It was not long ago that a 1.5 MOA capable rifle off the shelf was considered a good hunting rifle. Everything changes, and its ok. So is life. Some older chamberings worked great for their intended use at the time, but do not have what it takes to push their envelope nowadays. Back in the days, hunting was done on foot with iron sights. Rifles then only needed a certain amount of practical accuracy and the cartridges they shot were required to do their job at a much shorter distance than today.
This is why you may not see a 1874 Sharps in .45-70 or a Winchester lever action in .30-30 competing in a high-power rifle long distance tournament. These rifles are fine, their chambering potent, but they just cannot rival the modern rigs for that use. Older handguns typically have a thin front sight and a hard-to-find notch in the rear sight. They worked at the time and still do, but much better sights are now equipping modern guns, making it easier to acquire their mark and shoot accurately.
If you have an older gun, don’t try to turn it into something it cannot be. If you don’t care for the gun, sell it and use the money for something else or trade it in for another gun. If you are attached to it, let go of any frustrations it brings, and enjoy it for what it is. Like your friend or family member with their quirks and annoying habits, you may not want to go out with them all the time, but when you do, you always end up having a good time.