I have known her .... ages.
You can't use "since" there since "ages" is not a particular time. You can say "I have known her since we were both in kindergarten" but you can't say "I have known her since ages".
That is the last word. ;-)
'For' corresponds to the length of time and 'since' marks the beginning.
For answers 'how long' and 'since' answers 'when'.
'Than' is used in comparisons. 'Then' is not.
Further to previous explanations, 'since ages ago' would be fine, in my opinion. :)