it depends. There are several types of passives.
Most passives in natural conversation do not have overt agents, and most full passives are nonreversible, meaning that if the subject and object were switched, the resulting sentence would be unacceptable. A full passive such as you used is therefore quite rare. I don't know the exact age at which they're mastered, but I can tell you that even adults have problems processing them.
Susan D Fischer
Somehow, this query got buried in my email and I just ran across
it. Psycholinguistic experiments have shed a little light on this
question from a slightly different angle. Each language has what
is called a 'canonical order' for different types of sentences
and constituents. This is normally the most common order for the
type of sentence. Even languages like Latin or Spanish, which
have free word order, are subject to the canonical order in each
language. This has the following effect on your topic: in every
language, there are ways of focusing or emphasizing certain parts
of a sentence for different stylistic or grammatical effects.
Usually, this involves placing at least some parts of the
sentence in 'non-canonical' positions. In experiments with
various languages, it has been found that perfectly grammatical
sentences, but which are in non-canonical order, have a strong
tendency to be interpreted as if they were in canonical order,
and this happens in many cases even when pragmatic expectations,
the grammar of the language, and even common sense occasionally,
would be violated. And these experiments have usually been done
with adults, so it is no surprise that non-canonical orders,
whether the result of rules like the passive, or due to other
focus effects, can cause problems for children and other learners
of a language, and are, partly for that reason, acquired later in
the language-learning process.
James L Fidelholtz
As Professor Fischer points out, passive constructions vary quite a lot. This is true both within and across languages, which impacts age of acquisition including in child simultaneous multilingualism.
These two books have some discussion on variation and age of acquisition of passives in (mostly monolingual) child English:
Eve V Clark, First Language Acquisition
The Blackwell Handbook of Language Development