Suggestions are suggestions: you cherry pick them according to your needs and tastes. :) However ... where is your science? Simply "feeling" that these default values will not work for is not the same as actually testing them out.

use strict;
use warnings;
use Test::More qw( no_plan );
use DateTime;

for my $min (1 .. 20) {
    for my $max (30 .. 80) {
        for my $inc ( 0 .. 1 ) {
            is_deeply
                [ jeffa( $min, $max, $inc ) ],
                [ over2sd( $min, $max, $inc ) ],
            ;
        }
    }
}

done_testing();


sub jeffa {
    my $min = shift || 0;
    my $max = shift || 0;
    my $inclusive = $_[0] ? 1 : 0;

    my ($xs,$ns) = (DateTime->now,DateTime->now);
    $xs->subtract(years => $min, days => -$inclusive);
    $ns->subtract(years => $max, days => 364 + $inclusive);

    return $xs->ymd('-'),$ns->ymd('-');
}

sub over2sd {
    my ($agemin,$agemax,$inclusive) = @_;
    die "Minimum age omitted in DoBrangefromAges" unless (defined $age
+min and $agemin ne '');
    $agemin = int($agemin);
    $agemax = int($agemin) unless defined $agemax;
    $agemax = int($agemax);
    $inclusive = ($inclusive ? $inclusive : 0);
    my ($maxdob,$mindob) = (DateTime->now,DateTime->now);
    $maxdob->subtract(years => $agemin, days => -$inclusive);
    $mindob->subtract(years => $agemax, days => 364 + $inclusive);
    return $maxdob->ymd('-'),$mindob->ymd('-');
}
[download]

When i run this test i see no difference in the output of your code and mine. It is certainly possible that i have not tested as fully as can be tested, but i have more certainty that i what i provided will give you the desired output, as was my goal.

UPDATE:
You only need change one line in my sub. From this:

my $max = shift || 0;
[download]

my $max = shift || $min;
[download]

use Test::More qw( no_plan );

is_deeply
    [ jeffa( ) ],
    [ over2sd( ) ],
;

is_deeply
    [ jeffa( 10 ) ],
    [ over2sd( 10 ) ],
;

__END__
Use of uninitialized value $agemin in int at foo.pl line 37.
ok 1
ok 2
1..2
[download]

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)