Defining the age of headmates
This is a refined version of what Lettie wrote close to two years ago as of the writing of this version. It has been iterated on a few times, improved and a few of our thoughts have changed with time. This is what we now believe to be the best explanation of headmate age, but it is likely subject to change over time.
Age is not an easy thing to define for a headmate. Several different metrics can be applied with massively different results to the same headmate. For example a headmate may have existed in a system for 20 years, appear like a 6 yo but believe themselves to be thousands of years old but acts like a 16yo and now sharing the body of a 30yo. This leaves us with 5 very contradictory ages and no real way to judge which is correct. In fact, outside of a legal setting there is no right and wrong way to determine age and the system should be consulted about how they prefer to handle age and what a headmate can or cant do based on that.
The Five Ages:
Body age:
Body age is the simplest to define, it is simply how old the body is. Due to law being designed by a singletnormative society it is also the legal age of the system and it's headmates regardless of other preferences or ages.
Mental age:
This is how old a headmate acts. Much like a singlet can act older or younger then their age, so can a headmate act differently from their other ages. An important consideration is just because a headmate acts a certain way, doesn't necessarily mean they should be limited as if they are that age. It's not uncommon for a headmate to act childish (or sometimes even animal like) in order to cope with severe traumas but behind the facade be more then capable of adult choices. Care should be taken with mental age to ensure that headmate is not infantilise the headmate by assuming they aren't mature enough for certain topics.
Headspace age:
This is how old a headmate appears in headspace. Often linked to mental age but not always. Headspace age is a reflection of their form within headspace, not necessarily their mental capacity or actual age. For example a headmate may possess the body of a child in headspace but be much older and more capable then a child.
System age:
This is how long a headmate has existed within the system. While not a useful age for most purposes it can be useful for tracking the history of a system and many systems use this to determine the birthday of a particular headmate if known.
Exoage:
By far the most complex topic, a headmates exoage is based on their exomemories and can often have the largest impact on how a headmate acts. A headmate's exoage is based on their perceptions of their memories in headspace, linked to their identity within the system. For example a headmate may have existed for 20 years in a 30 year old body, the headmate may be an immortal vampire child with an 6 year old headspace form but act 16, yet to them they believe themselves to be thousands of years old based on their exomemories. Exoage can be difficult to understand, determine and confusing to apply.
Final comments and warnings:
At the end of the day, it's a system, in consultation with the individual headmate, that determines the age of a headmate as well as what they can and can't do. A headmate should not be considered a minor just because any one age in under 18. Doing so risks infantilising the headmate or the system and may discriminate against them unfairly. Its also worth noting that a headmate may grow older or younger, sometimes rapidly sliding between ages or moving at random in response to external stimuli, the mood of the headmate or simply because they decide they want to.