10.9 did introduce more strict controls over what is allowed to manipulate the GUI. Instead of a global "Enable Access for Assistive Devices" checkbox, you now need to allow access per application.
There's another thread here that details how to programmatically write in the settings to allow a specific application access to that. But since you're talking about an Applescript, not an application, I'm not certain if that's what's going for you. It could be you need to give Self Service access to the assistive devices controls. but in the interim, I would try this syntax instead and see if it works.
tell application "System Preferences"
activate
set the current pane to pane id "com.apple.preferences.sharing"
end tell
Also consider wrapping it up in a shell script instead of a pure Applescript. You may have better luck getting it to run.