What Age Does A Shih Tzu Puppy Get A Menstrual Cycle?

Oh, you mean heat.
It comes when the Shih Tzu is about 6 to 7 mths old

Dogs do not really have a menstrual cycle but they have oestrous (being in heat) instead, twice a year and from around 6 months approx( differs in different breeds and in individuals).  There is a saying that " Ovulation and oestrus go together but menstruation and ovulation do not".  That is precisely why the oestrous cycling and the menstrual cycling are very different. ///It is during oestrous( estrous) that canines ovulate and therefore become fertile and can conceive BUT human females cannot normally conceive during their menses . ( though there are some ladies who do, there are always exceptions to any rules!!)    The vast majority of( normal )human females ovulate 14 days BEFORE their next period and when no pregnancy occurs the whole hormonal reproductive system "tells" the womb lining to break down as there is no baby to use it and the whole process starts again every month from approx 13 to 50 years of age. .    So you can see how  very different the menstrual and the oestrous cycles are.   The oestrous cycle in canines has many different stages. ///  Plus humans can mate at any time but dogs can only mate at certain times during oestrus

Usually at 6-9 months of age but should not be bred in this time for she is very young.

5 months

Can be as early as 4 months or as late as 7 months but is usually 5 or 6 months for them. Here is the breed parent club and some other information for you.
www.americanshihtzuclub.org www.dogfoodanalysis.com www.irvinevetservices.com