Is there a good way to approximate alien biological age?

Question

Let's ignore extraterrestrial visitors on Earth because I don't want to discuss twin paradox, we should be focusing on life inhabiting on another planet instead.

Here on Earth we can do carbon dating because of plant and animal eat them even indirectly, it contains trace of C14, radioactive isotope produced as byproduct from the bombardment of cosmic ray, underground nuclear testing, etc. The alien sample should be exchanging C14 unless it dies and we know how to apply the known half-life of C14 to calculate their current approximate biological age, but does it hold true to other life in the universe as well?

Answer 1

First, a note - radiocarbon is useful for determining age as it is meant in an archaeological sense, the time since a plant or animal died. (And often was made into some human artifact.) It is not useful in determining age in the conversational sense of "the time since you were born".

The basic principle of radiocarbon dating will work on (some?) aliens, but with caveats.

First, it relies on the aliens' biosphere being carbon-based: in order to take up carbon-14 from the atmosphere, they need to use it in their tissue structure. All life on Earth is carbon-based, and there are some good chemical and physical reasons to suggest that it might be favorable in general, but non-carbon-based life is a longstanding trope in sci-fi, and they probably can't be carbon-dated. (There might be an analogue based on the properties of radioactive silicon or some such; I don't know enough about the chemistry of that situation.)

Second, the dating part of radiocarbon dating relies on a number of assumptions and references that are valid on Earth. The amount of total carbon-14 in the atmosphere varies on a geological timescale. Different types of plant life contain different amounts of carbon-14, and therefore so do different animals, depending on their diet. Traveling to an alien planet, you would be essentially starting over all of this reference work. Through comparison with other methods of dating you could compile reference materials for a new world, but it wouldn't be as simple as dropping the sample in your Analyze-o-Tron and knocking off for a cup of tea.

Answer 2

Depends on how old you are talking about. On earth, many organisms continuously deposit hard tissue of some kind, and the rate at which tissue is deposited varies annually based on the waxing and waning of seasons. Namely, in the winter/dry season (for the tropics), growth is slowed because of a scarcity of resources, whereas in the summer/wet season, growth is faster. This creates a series of annuli (i.e., "rings") in the hard tissues of many organisms, including the rings of trees, the number of lines of arrested growth in the cross-sections of long bones in most vertebrates, the shells of mollusks, the cementum of the teeth of many mammals, and the otoliths of fishes. Similar annuli form in the earwax of whales and the eye lenses of many vertebrates, which aren't hard tissues but grow in a similar way.

There's been a lot of press recently about Greenland sharks living in excess of 100 years, but the way that scientists figured that out was due to a chance occurrence. Namely, the fact that the eye lenses of Greenland sharks lacked an enrichment of radioactive isotopes after a certain point that was created by manmade nuclear testing in the 1960s, called the "bomb pulse". The researchers then backcalculated the age of the shark based on the amount of growth predating the bomb pulse to the growth after, which they could do because they knew how much had occurred since the bomb pulse. This wouldn't work for most organisms due to the way radioactive decay works, the lifespan of most organisms is so short that the change in C14 enrichment ratios from two decades ago to now is barely even noticeable.

Additionally, organisms are constantly taking in radioactive carbon isotopes through their diet, which is how it is possible to infer that an organism died so many years ago, death starts the radioisotopic clock ticking down because no new radioactive carbon is being incorporated into the organism.

What's going to work best for your aliens is going to be entirely dependent on your alien's physiology. What researchers would look for is a long-lasting, slow-growing tissue, one that is sequentially deposited and rarely if ever remodeled. For example, long bone growth rings and sometimes tree rings can be secondarily obliterated, making it difficult to back-calculate the age. Assuming a generally vertebrate-like body plan probably the first thing people would look for is the eye lens, which is pretty consistent in estimating age across Earth vertebrates. It may also work in cephalopods, but I don't think it would work in arthropod-like organisms because the eye lenses are shed and regrown with each molt. In fact, there really isn't any good way to estimate the age of lobsters and other long-lived arthropods for the exact reason that no hard tissue is retained between molts .