---------------------------                 
| now|unknown|factor| age |                  
---------------------------                 
|John|       |  0.66|     |                 
---------------------------                 
|Jane|       |  1.00|     |                 
---------------------------                 
---------------------------                 
|ago |unknown|factor| age |                  
---------------------------                 
|John|       |  0.5 |     |                 
---------------------------                 
|Jane|       |  1.00|     |                 
---------------------------                 
unknown$ = (7 - John's ago factor X 7) /     
    (John's now factor - John's ago factor);
for ago:                                    
    age$ = (unknown$ - 7) X factor$.