Hi there,
The amount of hours spent sleeping and the percentage of time spent in REM is highly VARIABLE; it also depends upon the individual. It does change across different stages of the lifespan. Generally, the amount of time spent sleeping gradually decreases as we age or get older. Also, the total sleep time spent in REM decreases hugely from infancy to adolescence and then remains relatively constant or stable into adulthood and old age.
To answer your question,
Infants: sleep 16 hours a day/ 50% in REM
Childhood: sleep for 12-13 hours a day/ 25-30% is in REM
Adolescence: 9 hours/ 2 hours or 20% in REM
Adulthood and old age: 6-7 hours sleep/ a third in REM (33.33%)
Just for your interest, the numbers and percentages were taken from the Macmillan psychology book and are only approximations due to the subjective nature of this particular topic. If you were to quote any of these hours or percentages in an exam, they will do!

Hope this helped!