The title is clickbait, as it isn't the lack of sex being complained about but that there was a scene of being happy and intimate that was cut. That scene can be done with or without the sex it isn't really important to showing them happy and together.

Ellie is 19 in last of us part 2 the video game. Idk how old the actors are but probably old enough for sex scenes just have baby faces.

Bella Ramsey is 21, so them looking quite young makes sense for him, though she has been in many productions from a young age. Isabela Merced is 23, so again, she is probably going to look quite young.

Nope they're adults