Many people find that their sleep behaviour changes as they get older. This is entirely natural and depends both on age and on a person's stage of life. As babies, we still sleep between 16 and 24 hours per day. Teenagers around the age of 14 sleep for 8-9 hours on average, while 70-year-olds spend just 6-7 hours in bed per night.
It is mostly the REM portion of sleep that reduces with time, i.e. dream sleep. This is due to a number of factors, including that as we get older we do not see as many new things in life, and our dreams reflect that.