TikkiRo,
Next time you fly, write a short concise letter to the Captain, stating who you are, any type of licenses/ratings you have, and that you would very much appriciate a jump seat ride.
In my expierence you will sometimes be allowed the whole flight including take-off, sometimes you will be invited in near to the TOD. Sometimes you will be denied a landing but allowed a look during the cruise. It depends on the captain and the company policy.
As a bench mark, these have been my personal expierences on international flights (and I'm only 17!);
Go Fly: Entire flight
British Airways: Entire flight
Buzz: Entire flight
Britannia: 1 hour during cruise and j/s landing
Ryan Air: Look during cruise and j/s landing
Airtours: Only a look - j/s landing denied (although I've heard from many people that I was just unlucky this time, most Airtours pilots, it seems, allow it.