How did Android 18 and Krillin have a baby?
Android 18 isn’t exactly an android, she’s specifically a cyborg. She was human once but Dr.Gero remodeled her and added cybernetics. Gero left her human enough to conceive a child. And so she did with Krillin.
Is Android 18 stronger than Goku?
Android 18 is very likely on the same level as Piccolo or maybe slightly stronger or weaker. Android 17 on the other hand is definitely stronger than SSJG Goku but not stronger than SSJB Goku.
How old were Android 17 and 18?
Assuming their number designation could be a hint to their approximate age when they were introduced, as they were kidnapped as children, this would make both Android 17 and 18 in their late 20’s or early 30’s by the end of the Dragon Ball Super anime.