Mine is a multi-part question regarding authentic Major League Baseball jerseys.

*What makes an MLB jersey "authentic"?  What makes a jersey a knock-off?

*Are authentic jerseys available in children's/youth sizes?  If so,
which New York City stores carry them?

My research so far:

*I have called some sporting goods stores, and I have been told that
since authentic baseball players are adults, authentic jerseys only
come in adult sizes; however, no one has been able to tell me with
absolute certainty that this is true, and I want to know for sure.

Thank you!

I browsed a few of the different teams here:

http://shop.mlb.com/home/index.jsp?clickid=topnav_teamTab_txt

I saw no children's sizes. Seems there are differences from team to
team as to what sizes are available to order.

That makes sense if you consider that each MLB team likely has their
own "authentic" jerseys mass produced in a single factory that they
have contracted for this purpose. That's what makes them authentic.
They *could* license others to manufacture them as well, but to very
exact specifications, to make them authentic.

~~Cynthia

Being a collector of Authentic MLB jerseys I can tell you a few things
to look for.  First of all currently as of this year the company
Majestic has taken over all MLB jersey manufacturing, however your
first sign is that on the sleve the maker of the jersey is sewn on and
not screened.  Russell, which used to for most if not all of the
1990's and late 80's, made both authentic and replica jerseys.  The
Russell logo which is an "R" was sewn on for both types.  However
their replica jerseys had their customary red and blue "R" while the
authentic jerseys had a single color "R" of the team that it was on
(ie home rockies jerseys had a purple "R").  You should also take note
that Rawlings for a few seasons until recently made authentic jerseys,
the periwinkle era Angels jerseys and Texas Rangers for example.  Once
again the comapny emblems were sewn on single color logos.

Another factor that I catch everytime are the front tags.  In the late
80's when Russell picked up the MLB jersey contract they emphasized
the front tag with their red and white logo and a green baseball
diamond with the size being a number.  NOTE this is very key to
knowing an authentic jersey by just seeing it, all authentic jerseys
MLB, NFL, NBA whatever are sized by number.  I have seen jerseys as
small as a 38, but the general scale goes 40/small, 42/medium,
44/large, 48/XL, 52/XXL, 56/XXL.  They go higher, but the point is
that they are NOT S,M,L,ect.  Replicas will always have lettered
sizes.

Finally I think I should mention the MLB logo on the back.  This isn't
a very strong indicator simply because I have seen the MLB logo sewn
into the back of replicas on ebay, but do remember that the
traditional red, white and blue MLB logo isn't used very often.  Some
teams use those colors because that's what their colors are, however
teams such as Tampa Bay have a green and black MLB logo to signify
their team colors.  The logo is always on the top of the collar on the
back.  The new MLB jerseys will have another distinct front tag that
has the word AUTHENTIC in red lettering with the MLB logo and
manufacturer of the jersey.

I hope this helps, its what I have learned from a few years of
collecting.  If you have any specific questions I might be able to
answer them.  As for childrens authentics, they are out there.  I know
there are youth authentics on ebay, but not toddler, or at least I
haven't been looking.  I did learn something after seeing and wearing
many different jerseys is that especially the older jerseys are made
heavier with more of a non breathable cotton that todays high tech
light weight polyester.  Let me know if you need anything else.