In the future how would astronauts compare ages from different solar systems?

Excellent question. I think it will require a new type of time measurement. We’re going to have to imagine a different constant to use to gage our age by. Perhaps we can use the speed of light. That is a universal constant across all areas of the universe.

Stardates, obviously.

I am 5 yrs old in Jupiter years

Also would have to consider relativistic effects

Age is measured by the passing of Terrestrial Standard Time which is based on the meridian at Greenwich. Allowance must be made with regard to the effects of time dilation in a society where near light speed travel is commonplace. To prevent “time crimes” or faking your age, everyone must wear on their forehead a unit which displays their precise date of birth relative to Greenwich Meantime in Terrestrial years and milliseconds as well as their precise age.

These “time unit” displays are bolted into the bone of the wearer’s skull and the penalty for removal is pronged torture and death. Tampering with these devices is the most serious of all crimes in the Galactic Federation and punishments are not specified but are devised on an individual basis by a panel of judges.

There’s already a non-astronomical time measure, for a very long time now. It’s chosen to correspond to Earth astronomical time, but a second has long been defined by physicists in terms of atomic vibrations, rather than a fraction of an Earth rotation.

Link The basic idea, as far as I can see, is that caesium atoms go back and forth between two states with a very high frequency. The time it takes to go back and forth some fixed number of times is defined as a second. The measurement error is one second in 300 million years.