There seem to be two types of age in the question. The first is how someone is in terms of how long they have lived or their age by the calendar. While the second age is their biological age or equivalents thereof. This would be a sort of gerontological clock.
For interstellar colonies, calendar age can be calculated both in terms of the colony planet's local calendar (or Colony Planet Years) and in Standard Earth Years. @knowads correctly suggested calendar age can be calculated in seconds. The second is a standard unit of time which will remain so irrespective of the length of the day on any given planet or the period of its orbit around its primary star.
A 32 year old (in Standard Earth years) is 32 times 31,536,000 seconds old or 1,009,152,000. This is, of course, a ridiculously exact figure. So expect persons communicating across the colonial empire to give their calendar ages in approximations. Like "On May 23, I turned 1,009,152,000 seconds or 32 Standard Earth years old." The person receiving this age can quickly estimate the age of their pal on another planet.
For biological age, it's necessary to make one assumption. Namely, that medical and biological science has advanced sufficiently that there exists a simple test for estimating how far along someone's lifespan any given person is gone. This doesn't mean when the person reaches the end of their estimated lifespan they drop dead immediately. This will always be a probability.
Assuming two people chatting via ansible (don't worry this is only to simplify the reasoning here) and they both have had the lifespan test. "I'm 32% of my lifespan long," she said. "Oh," he replied. "I've just reached 31% myself."
Percentages are one way to calculate an estimated passage along a person's lifespan. It could be done by the time on a clock face. Any suitable metaphor will do. If the test worked sufficiently well people could talk about how many more years they expected to live.
"I'm 400 now and I've only got another 800 years left." This will need to be translated into whatever calendar the speakers use to calculate age. This can be Standard Earth Years or their respective Local Colony Years.
How does the biological age test work? Perhaps by the length of telomeres or any other molecular biological clock that may be ticking off our lifespan. There might be ways of measuring the accumulated wear and tear on cellular structures. The biological sciences are simply too complex to predict where they will go or what they will discover. For this reason, it is simpler to assume that a biological age test will exist in the age of interstellar colonization and leave it at that.