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MangaIsGood
When he's having trouble in the bathroom, she goes in, and we can see (what looks like) his dick reflected in her eyes.
Then, the elevator floor counter panels (1,2,3,4,5,6...) are meant to reference them having sex several times in a row.
"I'm back in my 20s" is basically saying they had so much sex it felt like they were younger again, just like the special ingredient was supposed to do in the end.
Assuming Kirara said "Freedom!" because she'd heard Oku-san through the door or saw her washing dishes, then what she's doing on the last page is probably an imitation of Oku-san having sex (whether she just heard her voice or actually saw it, your guess is as good as mine).