Second answer, based on the OP's comment to Techidiot's answer:
I am very old -- hundreds of years
Most languages, except constructed ones like Esperanto, are hundreds or thousands of years old.
Reborn now and then, I have many peers
Dead languages can be 'reborn', like Latin or Cornish. Nearly every language has many relatives in the language tree.
Have thousands with me - you know many of them
There are thousands of languages in the world.
They all can change-- and yet stay the same
Languages evolve over time, but most of their structure stays the same - we can still understand Shakespearian English today.
Sensible and orderly-- you all know me
Languages have some logic and system to them, and everybody speaks at least one.
Believe it or not-- I am inside me.
Every language has within it a word for the language itself: English, Francais, Deutsch, Espanol, ...
Hope this riddle has not been covered already.
Not that I know of.