In plenty of stories you have beings who age incredibly slowly and are technically immortal. I have a handful of similar such beings who have through some means or other attained the benefits of healing blood which lead to their 'immortality'.
Now, I've not made these people unable to age at all. They do age, just incredibly slowly. So eventually they will grow old, and eventually they will die. Probably something to do with a combination of their cells telomere lengths and the healing aspects of their blood thinning out... but they will live for an incredibly long time, a couple of centuries to a millennia or two at my last count. And seeing as the fundamental role of life is to go forth and multiply, I have allowed them to have children, albeit very rarely and very slowly. It's actually quite a sad affair involving normal 'mortal' women, as immortal women do not have their period for centuries. The universe is not going to get overrun with immortals.
My problem is, how do I get the newborn immortals to grow into adulthood without subjecting them to several decades to centuries worth of childhood and worse, puberty. Puberty is tough enough as it is without being stuck in it for decades. I don't mind them taking several years to age a 'normal' year but I want to avoid decades to age a 'normal' year. I am trying to avoid the 'wise child' cliché, as well as resorting to birthing a fully grown adult. As in 'The Birth of Venus' or Athena.
How can I explain why my immortal children age faster than the immortal adults?
Their bodies heal all wounds and infections, their cells renew continuously without cancer forming. If a limb is chopped off, they can reattach it (if quick enough) but they can't grow it back. And if they break their bones and let them set in the wrong position, they will heal in the wrong position. Stuff like that. I'm not too worried about this aspect of their immortality as it is pretty well covered in many stories and even other questions on SE.
I've figured a way to ignore/handwave away the whole 'pregnancy is essentially an infection' issue by making the immortal women sterile. (EDIT actually they are fertile and develop as any normal female, their bodies immune system is just so keyed up that it rejects all foreign genetic material and either never conceives, or aborts the foetus very very quickly. END OF EDIT). 'Normal' women with some limited exposure to the source of the immortality (but not themselves immortal) are involved and the foetuses develop over several years rather than 9 months. FYI this is more an incubation in an external pod type pregnancy and there are no hormonal women walking around who have been pregnant for several years. It's also not about extended growth inside a womb but rather just very slow foetus development inside an artificial womb made from the same source as the adapted/healing blood. So they are still born at a similar development stage as a normal human baby due to normal energy and child development constraints etc. Again, I'm not too worried about this aspect either, unless I should be.
No magic, no technology, just adapted blood and a naturally grown birthing chamber/cocoon/pod like a silkworm cocoon or leaf pod.
I'm looking for a logical and semi reasonable explanation for increased rate of aging for 'immortal' children compared to the much slower adult aging rate.