I'm under 24 and it almost kept me from getting this job. Having friends, and 5 years real world exp helped though. Honestly for Linux most real big companies still aren't all the way there yet because its still new and its hard for them to justfiy using it. They still use a lot of big iron stuff ( Solaris, AIX, hell even HP-UX ) because it has nice support and they have someone to blame if something bad happens. So I wouldn't count those out in your learning of UNIX operating systems.