i guess unfortunantly, I'll answer your question with a question!!

why those two particular parameters?
in that order?
(i think it would help in giving advice)
late 20's with children ,, low thirties without?
when you understand the russian culture, youll realize a few things that cut against those two parameters..and thier order (in generalities of course)
*shrugs*
i really hate making generalizations on an entire population,
as you can always find more excepotions than rules when it comes to human beings..LOL
that said:
RW are generally .,if married, going to have children,
it is more the whole point of the marriage , than it is in the west..
(not the entire point,, but you get the fundamental idea, and if you dont get it yet, her mother or grandmother will fill you in mister!

)
and they are generally married by EITHER of the ages you dictated-
or they are felt to be old maids
(again in general , and yes there are plenty of exceptions)
add those together and you have
Low to mid thirties WITH children would seeming to be the most common and in your suituation most reasonable, but you intentionally excluded it,..
i dont mean to offend, ,but it appears
from how the ages are presented ,,
you maybe willing to accept an additional child "if' the woman is younger
and not so accepting if she is little older..
thats certainly how the often very pragmatic RW would view your question..
and how its worded. .(and again *in general* they would read far more into it,
and its wording, than a silly western man like myself LOL)
Honestly if you have the 3 kids often, and involved heavily in thier lives,
i advise you look at RW with children already,,
and as Lily said its thier personality that will matter ,not thier age..
and late twenties to your own age of 40 is a reasonable age group ?
For some background, I'm your age,have a decent or at least fundamental understanding of the russian language , very happily married 4 years , my wife is 29 now (had no children, but wants some very much) and i have a now 11yo boy that i've always had sole custody. She is an absolutely amazing mother,
I'm incredibly proud of her and him..but if she had prior experience of a child of her own, it would have made blending a cross cultural family easier on all involved.
your RW mileage may vary, but i doubt it..
my two kopeks!..