Middle distance covers the 800 metres and 1500 metres track events. Comparing past and present world record holders it would appear that 800 metres and 1500 metres male athletes are most likely to peak around the age of 25 and female athletes at 27 years of age.
http://www.brianmac.co.uk/middist/index.htm
http://www.brianmac.co.uk/middist/index.htm