A powerful human king had a child with an elf princess. His life expectancy is about 70 years (human average), hers is 2000 years. The half-elf prince is expected to live for about 1000 years, more or less the average of his parents life expectancy.
Years later, the prince has two children: a boy, with an elven lady; and a girl, with a mortal woman. Both of them, having a half-elf father, are gifted with a long live, but the boy lives longer than his sister, since he's the son of an elf and she's the daughter of a mere mortal.
The prince becomes king, and his son becomes king after him, and so on. Their descendants have children with elves and also with humans (increasing and shortening their lifespans respectively). After generations of mixing with humans, the powerful half-elven kings lose the grace of the magical folk and become simple mortals, with an average life expectancy.
What I've been doing until now is simply calculate the average and give them linear aging. But this yields certain problems... The prince's son, for example, would have a life expectancy of (1035+2000)/2 = 1517.5 years. With linear aging (0 looks 0, 1517,5 looks 70-80), this poor boy would be over 300 years old before even reaching puberty. If the same happens with pregnancy, he would be in her mother's womb for over 15 years!
So, I want to know if there's a mathematical formula I can use to calculate their aging rate taking into account that they should reach puberty relatively early on their lives and lengthen or shorten their life expectancy depending on whether their mothers are elves or mortals.
PD: Of course all of this is fantasy and I can give them the life expectancy I want, but I'd like to keep it more or less realistic. Also, having a "consistent framework" to work within makes me feel more confortable creating. Thank you very much!