First of all, suspenders are actually age-reducing artifacts. There is no age difference for girls. As long as you put it on, you will look young and simple. The sling gives people a sense of campus, which always makes people feel as if they are in the environment of youth campus.
Moreover, the sling is also a matching artifact. If you simply put it on casually, it will look very pure. Pants are easy for people who are fat, thin or tall, and they don't make people feel embarrassed.
So I think suspenders are always out of date!