I have read many publications regarding the ability in a species to identify other conspecifics according to their relatedness (relative or non-relative), sex (male or female), reproductive status (in oestrus or not).
I have also read publications where age has been demonstrated to bear important social significance indicating of reproductive quality (both in terms of more experienced and higher quality older mates and in terms of senescence), competitiveness (e.g., in foraging, defense, group leadership), predation risk (accepting certain age groups into the social unit might increase or decrease predation risk) etc.
It seems that age should be an important factor in determining life strategies but I do not currently recall that many publications where the ability by a species to identify the age of conspecifics has been analyzed.
There have been studies regarding, for example, red deer female selectivity which favours older males (better quality, lower chance at suffering injury during copulation) and which avoids young, inexperienced males.
Also, there have been some references of the aforementioned predation risks in groups with young vs. without young and in groups with adult males vs. without adult males.
I suppose that an easily recognized age group threshold is sexual maturity and that species can probably tell apart sexually mature vs. immature individuals, a life stage which is often correlated with age.
In the wild, few individuals reach a very old age and menopause (another threshold of sexual potential) has been observed very rarely (e.g., in chimpanzees after the age of 50).
However, a decline in sexual activity is probably observed in a greater number of species after a certain age even if the individual is still capable of mating and conceiving.
In fact, the short lifespan in the wild probably makes age-related studies difficult because the sample size in older age groups tends to be small.
However, there are long-lived species and I believe it would be interesting to see whether animals, too, differentiate between ‘age groups’.
In our societies, we have formed concepts of such groups based on biology, health and social status, professional investment, legal liability (‘decision-making proficiency’) etc.
It would be curious, for example, to determine whether animals also have created more abstract concepts of age (clumping individuals of a certain count of years in one category) or whether their age-based decision-making mostly relies on active functions.
For example, if there is a species where mating with partners that are too young or too old is selected against, it would be interesting to analyze whether these choices reflect more conceptual, generalized categories (e.g., individuals aged 2 – 4 or 10 – 13 etc.) or if they closely follow some physiological, chemical, hormonal etc. cues that are only correlated with age (but in case the individual’s active functional potential does not correspond to their age group, the potential and not the age group is considered by the prospective mate).
It would be similarly interesting to see if individuals of different age elicit different responses to, e.g., scent-marks left by them (having accounted for dominance status which often correlates with age).
Also, extraterritorial forays into neighbouring territories might vary according to the age of the neighbouring territory holders.
If any such detection mechanisms occur, it could be studied what cues are used to determine age in conspecifics (e.g., vocalization pitch in red deer).
Notes by Ieva 