Sources and method
Ages come from Voteview's HSall_members.csv, one row per member per Congress since 1789, presidents included. Voteview records a birth year, not a date, so an age here is the age a member turns during that calendar year: right for anyone whose birthday has passed, a year high for everyone else, and about half a year high on average across a chamber. That is precise enough for medians and trends and too coarse for any claim about a particular person on a particular day.
Each Congress is spread across both calendar years it occupies, so a year shows that Congress's full roster — a seat that changed hands mid-term shows both holders in both years. A president who left in March belongs to the previous Congress: 1841 seated three presidents, but Van Buren sits with the 26th Congress and only Harrison and Tyler appear here under 1841.
Birth years are missing for 94 of 12,635 legislators, who are left out of the medians and counted beside them. Four presidents — Washington, John Adams, Taylor and Hoover — have no birth year on any Voteview row, and a fifth, Cleveland, carries one on his first-term rows but not his second. Cleveland's own value is filled in from those rows; the other four use uncontested dates from the standard presidential record and are marked † wherever they appear. Every other file in this lab keeps Voteview's blanks untouched, so its validation output still reports the gap.
Washington and Adams are also why a birth year is slipperier than it looks: both were born under the Julian calendar, and Washington's 22 February 1732 New Style was 11 February 1731/2 Old Style — a different year, from the same birth.
Territorial delegates appear only from the 113th Congress, which is how Voteview carries them, so House counts sit at about 435 throughout.