I watched itsaw on TV laterecently that old fashion has become popular among young people. I saw on TV recently that old fashion has become popular among young people.

OR "I have lately been seeing on TV that old fashion has become popular among young people". First one - you're referring to a specific incident; second one - this is a general observation.

It has been often said "fashion goes around", so this trend might be temporary as alstoo. It has been often said "fashion goes around", so this trend might be temporary too.

I like casual fashion ordinarily, but I would like to try amore mature fashion. I like casual fashion ordinarily, but I would like to try more mature fashion.

OR "I usually like casual fashion, but I would like to try more sophisticated fashion too". I think the word you're looking for is sophisticated, because 'mature' fashion, while not really a thing calls to mind things a middle aged woman who is not particularly fashionable would wear, which doesn't really make sense to compare with casual fashion as it is also casual. 'Sophisticated' is more of a natural contrast with 'casual'. You could also use 'intentional'.