Based on Puzzled Pig's answer, are you
a set of encyclopedia
I was the child of a ruler, but I wasn't human
one of first general knowledge encyclopedias was written by Pliny the Elder
All of my kind shared the same, strange weakness.
All books are subject to damage from fire/water/age
We loved to be kidnapped by others.
An encyclopedia was a large/expensive purchase. It was common to be purchased by a group for group usage. Or borrow the ones from the library.
We would speak all of our secrets to them.
people read to learn all the information contained.
Some see it as a weakness, but to us, it fulfilled our purpose.
the purpose of an encyclopedia is to relay the knowledge it contains.
I was the best at it.
Encyclopedias were best at relaying knowledge because they contained information on many topics.
But eventually, my speech began to fade.
Three reasons here: 1) the ink would fade. 2) the knowledge would need updated. 3) the hardcopy book format getting replaced by the internet/online resources.
It inhibited me from fulfilling my purpose.
no longer able to pass along information when 1)the ink is faded out. 2)the knowledge is out of date. 3)the internet/online resources make hardcopy publishing no longer economically viable.
Many in my kind had kept themselves young.
Publishing new versions/yearly "Year Book" updates/Moving to Online/CD/DVD versions.
But I, had let age defeat my purpose.
didn't transfer to online/digital publishing and went bankrupt.
My son carries on my purpose now.
But I know even my son will fade just as I did.
Everything changes. Something will eventually replace wikipedia.