I think the best way to approach this is to parcel out certain mental and physical characteristics that are part of a race's actual character mechanics, and use that to determine how they operate at different pre-maturation age brackets.
So rather than having an elf baby who is physically an infant for a hundred years, you have multiple periods of adolescence and quasi-puberty that each serve as a segment on the road to full maturity.
For example, Elves. Let's say that elves finish developing physically and mentally by the age of around 75 years old- and have a lifespan measured in the hundreds of years, capping out somewhere around 8-900 years or so on average.
A newly born Elf developes physically very similarly to a human child, if a bit slower, but stays in the innocent state of a young child (like a 5-10 year old kid) until their first maturation hits at around 15-17 years of age. While they might be similar in size to a human child of 15-17 at those years of age, they are mentally much more like children of ages 5-10. The world seems to them to be much simpler, they aren't capable of making complex long term decisions, weighing the consenquences of their actions, and haven't really developed the ability to conceptualize that other people have their own existences properly. Everything they do is through the lens by which they experience the world.
While a human is generally considered mature around ages 20-30, an elf retains the child-like innocence at least until the age of 25 before he or she becomes mentally developed enough for puberty to set in.
While a human experiences puberty all at once, manifesting as a whole slew of physical and mental changes, their bodies becoming supercharged with hormones and their emotional state of being and self-conception becoming extremely unstable for the duration, elves progress at a much more languid pace.
The initial change elves experience is the increase in physical stature. They grow taller and hardier, but do not yet develop primary sexual characteristics. This occurs from the age of 25 to 40 or so, and an elf at this point in their life cycle is easily identifiable by their seeming adult proportions, but lack of any obvious hanges to their body structure to favour one or another sex, excepting whichever characteristics they possess by birth and family.
During this period, the Elf also developes an increased awareness of the world, and begins to develope the ability to properly conceptualize the existence of other people. However they are also extremely vulnerable to outside influence during this period, and it is extremely rare that any elf at this age might be found outside their family communes. Elvish communities believe that this is the point in which the individual begins to truly develope their own self identity, and thus this time is spent studying history and acquiring knowledge so as to have as much available resources to make an informed decision.
An elf who is not given the space, isolation, and information to decide who they want to be when they achieve full maturity will make their decision regardless, based on personal experience, social connections they have established with others, and often may be coerced into developing negative or harmful personality traits by another individual who maintains access to the elf during this period. While it is the duty of an individual's parents to instill in them both knowledge and wisdom, not every individual benefits from a stable family or community to do so.
It is from the age of 40 to the age of 65 that an Elf begins and concludes the development of both primary sexual characteristics, and the mental development necessary both for full maturity and informed self-determination. These changes take a different amount of time depending on the individual, and may be hastened or slowed based on environmental factors. While humans experience the desire to copulate with anything that moves during their development of primary sexual characteristics, an Elf instead developes exceedingly strong emotional bonds with individuals in their community, pursuing their first romances at a more sedate pace as they become aware of sexual factors both in themselves and in others. As the individual passes through this phase of development, they lose the wide-eyed innocence of a child, and begin to develop the more cognizant outlook of an adult. While their overall personality is dependent on their earlier development, and an individual may remain very innocent in manner and thought well through adulthood, it is during this period that they grow out of childishness, learning to take responsibility for their actions, and gaining an understanding of the temporal nature of many interactions. It is often at this point that a developing elf first sets out into the world, after being judged ready by their parents or community.
Once physical and mental development is largely complete, a young elf will need to gain experience from direct exposure to the outside world. Often partnering with adult adventurers from elven communities, these elves begin to develop the necessary skills for adult life outside their communities, learning how to interact with other races, and use skills that are often unique or derived from those other races. While every Elf possesses some training in their chosen vocation as part of their upbringing, it is during this period that they crystalize their understanding of that training and study, and begin walking their own path. In doing so they finally gain access to their ancestral ability to resist sleep effects of supernatural origin, and a natural resistance to enchantment effects.
Elves at this state of their upbringing are identifiable by their relative inexperience, but by a self confidence that younger elves lack, and the talents of having trained in a proper vocation. While elves during the exploitable middle puberty are often possessed of no relevant skills for survival in the outside world, these elves are reasonably competent. Though they may be brash, and quick to action or emotion, these are traits that will fade as they pass through the doors into adulthood.
Basically, elves start out with just -constitution, then develop first +dex as part of 'middle puberty', then +intelligence and +perception as part of 'late puberty' or 'second puberty'. It is during this second period of development that their immunities 'come in', and then bamf, you have an elf who is around first level at the age of 65-70.
