I feel I should sit on the fence and add a couple of comments here... The half life of tritium is an indisputable fact - after 12.3 years, the tritium gas held within a vile will be emitting exactly half the radiation as it did when new. So the vials will dull over time - note that if you look at items like the tritium powered exit/emergency signs, they are sold based on the lifespan of the tritium. However, not all tritium vials are created equally - different sizes, different phosphors (the bit that actually does the glowing), and different amounts/pressure of gas within will all change the output. So comparing two items will be very hard given the timespan between their manufacture. I would hazard a guess that if you are comparing a 20 year old watch with a brand new one, it is entirely possible that they are of an equal brightness - but this will also come with the note that the 20 year old watch probably had brighter markers when it was new than your new watch does. To go alongside this, I have compared multiple 23x3mm tritium vials against each other - different colours are of noticeably different output, but also two theoretically identical green tubes had different outputs, so there definitely can be a change - as I noted above though, as I didn't get them both from the same place at the same time, I couldn't say whether the one I know was secondhand was a few months or ten years old, or when exactly the ones I have bought new were manufactured, so whether the difference was ageing or another factor I do not know.