Which is Italy's leading Fashion Design School for someone around 20
years old who want to train for the top end of the market?

Hello.

Istituto Marangoni in Milan is widely referred to as Italy's leading
fashion design school.

Istituto Marangoni
"The Leading Fashion School in Italy"
http://www.istitutomarangoni.com/

For an overview of the history of the school:
http://www.istitutomarangoni.com/en_intro.htm

For other information, visit:
http://www.istitutomarangoni.com/en_segreteria.htm

Here's what other web sites say about the Marangoni school:

From Hong Kong Apparel:
"For over sixty years the prestigious Instituto Marangoni of Milan,
Italy's leading fashion school has trained thousands of fashion
professionals with many of their graduates going on to work in famous
name fashion houses including Dolce & Gabana, Versace and Moschino. "
http://www.hkapparel.com.hk/announcement3.htm

From Lybot.com:
"Istituto Marangoni is the leading Fashion School in Italy. "
http://www.lybot.com/Fashion/Alphabet/I_1_25.htm

From Ambitalia:
"The leading fashion school, located in Milan"
http://www.ambitalia.org.au/iyg/categories/italian_products.htm

Another school that is sometimes mentioned as a leading Italian
fashion school is Istituto di Moda Burgo, also in Milan.
Visit the school's web site at:
http://www.imb.it/

Here's what another web site says about Istituto di Moda Burgo:
"Istituto di Moda Burgo, one of Italy's leading fashion design
colleges."
http://www.abirdseyeviewof.com/Italy.html 

Made-in-italy.com offers a list of some other fashion schools:
http://www.made-in-italy.com/fashion/schools/

search strategy: "leading fashion", school, italy

I hope this helps.

For anyone that might still be interested in this question/answer:

Also take a look at 
www.Polimoda.com <http://www.Polimoda.com>

"Generally considered the best school in the sector [in Italy]." Il
SOLE-24 ORE, 23 Luglio 2001  - -  "Sole24 Ore" is the main financial
newspaper in Italy.

From the site:
"Polimoda International Institute of Fashion Design and Marketing
offers specialized courses in fashion design, production, sales,
marketing and merchandising sectors of the fashion industry in
Florence, Italy."

The school works in collaboration with the Fashion Institute of
Technology at the State University of New York, which is also a
leading school in the sector in the US.