Condensation in 20 year old double glazed windows - possible to replace glass or seals?

You can usually replace the glass however the frames being twenty years old will probably not allow for the typical double glazed standards used today, they are typically thicker and more energy efficient. It may be beneficial to carry out full replacements.

There are plenty of companies that specialise in affordable replacement units, ideal if your frames are only a few years old. As your windows are 20 years old, it would be more cost effective to change the complete window as you will benefit from increased security, energy efficiency and be able to update the look of your home.