score:5
Everything is free if you are under 26 years old, also the Trianon.
There is just an exception: you can't visit the gardens when there are the musical fountains shows.
As it says on the ticket shop page:
Attention, on days with Musical Fountains show or Musical Gardens, access to the Gardens is free only for children under 6. Tickets for the Musical Fountains Shows or the Musical Gardens can be bought online or at the entrance of the Gardens.
If you are under 26 you can get a reduced rate by buying tickets for the musical show and clicking "reduced rate" when you choose the number of tickets to buy.