I think that if the age of consent is gradual, the age difference allowed should too.
First scenario, two 15 year olds decide to start their sexual life together. They’re fully informed. Nobody is taking advantage. It’s very different to a second scenario where a 17 year old is dating a 36 year old.
It’s funny because the problem is usually described with terms these specific French philosophers used: power dynamics. It is too unbalanced. The adult has way more power than the teenager and that’s not healthy, it can even be dangerous.
I would argue that we should be careful with age differences until our early-mid twenties, even if the law gives us a free pass from our 18th birthday. But, anyway, yeah, in the second scenario the teenager is older than the first ones, so we’d assume that if the first ones were okay the second too, but the age difference matters IMHO.
Aliens are here to unite us (/s).