Trying to sort on two fields, second then first

A key specification like -k2 means to take all the fields from 2 to the end of the line into account. So Villamor 44 ends up before Villamor 50. Since these two are not equal, the first comparison in sort -k2 -k1 is enough to discriminate these two lines, and the second sort key -k1 is not invoked. If the two Villamors had had the same age, -k1 would have caused them to be sorted by first name.

To sort by a single column, use -k2,2 as the key specification. This means to use the fields from #2 to #2, i.e. only the second field.

sort -k2 -k3 <people.txt is redundant: it's equivalent to sort -k2 <people.txt. To sort by last names, then first names, then age, run the following command:

sort -k2,2 -k1,1 <people.txt

or equivalently sort -k2,2 -k1 <people.txt since there are only these three fields and the separators are the same. In fact, you will get the same effect from sort -k2,2 <people.txt, because sort uses the whole line as a last resort when all the keys in a subset of lines are identical.

Also note that the default field separator is the transition between a non-blank and a blank, so the keys will include the leading blanks (in your example, for the first line, the first key will be "Emily", but the second key " Bedford". Add the -b option to strip those blanks:

sort -b -k2,2 -k1,1

It can also be done on a per-key basis by adding the b flag at the end of the key start specification:

sort -k2b,2 -k1,1 <people.txt

But something to bear in mind: as soon as you add one such flag to the key specification, the global flags (like -n, -r...) no longer apply to them so it's better to avoid mixing per-key flags and global flags.

With GNU sort you do it like this, not sure about MacOS:

sort -k2,2 -k1 <people.txt

Update according to comment. Quoted from man sort:

   -k, --key=KEYDEF
          sort via a key; KEYDEF gives location and type

   KEYDEF is F[.C][OPTS][,F[.C][OPTS]] for start and stop position, where
   F is a field number and C a character position in the field; both are
   origin 1, and the stop position defaults to the line's end.