What were the main topics about which Oscar Wilde wrote? Was he interested in youth and the concept of staying young and beautiful?
The main topics about which Oscar Wilde wrote involved the conservative Victorian social mores of the period in which he lived. His most widely-acclaimed work is the play "The Importance of Being Earnest," which portrays such social mores in a farcical light, casting as many aspersions upon Victorian England as he could. Who but a living manifestation of Victorian England could possibly desire cucumber sandwiches, as one character in the play did, and the emphasis throughout the story on associating with one's own class. Wilde's disdain for the hypocricy he observed in England's upper class was evident in his statement that "every saint has a past and every sinner has a future."
Not all of Wilde's depictions of Victorian England were satirical, and not all addressed the social mores he found suffocating. His greatest influence was perhaps as an aesthete who believed that art need not serve any purpose other than to be art, a heretical perspective given the social and political constraints that influenced much European art over the centuries. His only novel, the decidedly serious The Picture of Dorian Gray, placed a painting at the center of the story of a particularly vain man who dreads the notion of aging while his portrait remains a daily reminder of how handsome he once appeared ("In the center of the room, clamped to an upright easel, stood the full-length portrait of a young man of extraordinary personal beauty...").
To the extent that Wilde could be considered to have been interested in beauty and youth, then the answer perhaps can be found in The Picture of Dorian Gray, which also draws attention to his views on art, as mentioned above:
"The artist is the creator of beautiful things. To reveal art and conceal the artist is art's aim...Those who find ugly meanings in beautiful things are corrupt without being charming. This is a fault. Those who find beautiful meanings in beautiful things are the cultivated. For these there is hope. They are the elect to whom beautiful things mean only beauty."
Wilde's interest in physical beauty, and in the role of art -- to the extent such a role exists -- in portraying beauty, is central to much of his work. His comment that "it is better to be beautiful than to be good" is entirely consistent with his personal philosphy of life. For an individual so refined and so enamored of physical beauty, the two years he spent in prison for the "crime" of homosexuality had to have been physically as well as emotionally devastating. That he died poor only a couple of years later was perhaps testament to his dependence upon the kindness of strangers ("True friends stab you in the front.")