לז"נ הקדושים, מלכה בת ר' מאיר שלמה ואת שני בניה ,ניסן ופנחס הי"ד יום השנה הוקבעה לכ' סיון

For part 1 see here 



Part 2

R. Chaim Brisker zt”l said, "a Choleh who is not obligated to fast, has no reason to be Machmir and fast. " 

R. Chaim explained, Since  Cholim were never part of
the Gezairah( to fast). Chazal were Gozer only for healthy people to fast.

Therefore there is no point  to be Machmir . Had Chazal included everybody in the Gezeiras Ta’anis and then exempted  the Cholim, then there would be an inyan,
(only when possible and no harm), to be Machmir and fast .

Perhaps we can say the same Sevarah (logic) for someone who is underage (15-18 years) on the fast of 20^th Sivan. Being those under 15 &18 were never included in the Gezeirah, there is no point, even for over 12 – 13 olds (after Bar/ Bat Mitzvah) to be Machmir and fast.

The 4 fasts where children do fast, is only because of Chinuch. Whereas  20^th Sivan fast starts at ages 15 & 18, Chinuch is not applicable after Bar/Bat Mitzvah. כנלע"ד