Definite adjectives

Dear all,
I was under the impression that "definite adjectives" i.e adjectives
with a postposed (and later fused) personal pronoun, like in OCS, was a
Balto-Slavic innovation. In fact one of the main arguments for a genetic
Balto-Slavic subgroup.

But then I read in _The History of the Lithuanian Language_ that
1) This cannot date back to Proto-Balto(-Slavic) because of the
(existence of) psotposed locatives in -en in lithuanian that are attached
to the basic adj. stem as well as the following pronominal element. But I
don't follow this argument - couldn't that have happened after the breakup
of Proto-Balto-Slavic but still before the fusing of the adj. ending
and the pron. element in Lithuanian (or all of Baltic)?
2) "definite adjectives" appear in Avestan as well but were not
continued there

Any light on these issues are appreciated. (The similarity in Germanic can
of course not be genetic).

all the best,

Harald