The usual form of the question is "what was the age at which each became President?" (ignoring any distinction between those who were elected or those who took over at the death or resignation of their predecessor). Including ALL Presidents, and counting those later elected "in their own right" increases the age a bit.
The median age overall (whether elected or not) is 54 years, 11 months.
Most by far were in their 50s (twenty-four), esp. 50-55 (fifteen).
The range of those elected has been from 43 (Kennedy) to 69 (Reagan); Theodore Roosevelt became President at 42 (on the death of McKinley). No others under 45 have been elected, and only two were over 65 when elected (W.H. Harrison at 68, and Reagan), so we could say "mostly between 45 and 65".