How rephrase the following sentence so that it can be understood by a 13-year-old boy?
Pocket squares are a standout amongst the most vital apparatuses for a man of style. It's important to have the correct pocket square for the correct event.
To simplify the sentence a good method would be removing or replacing the words that are more difficult for younger readers.
You could use the SUBTLEX-UK corpus. It is slightly more accurate in predicting word recognition than other corpora such as the British National Corpus, CELEX, and the Google corpus. More importantly, it includes columns with word frequencies derived from subtitles of television programs for preschool (i.e. CBeebies), and primary school children (i.e. CBBC). A Zipf-scale gives a natural frequency ranking from 1 (low frequency) to 7 (high frequency). For example, the word elephant has a CBeebies Zipf-value of 5.23 and a CBBC Zipf-value of 4.76. You could look for words with a Zipf-value below 3 and replace them. Though you should use your intuition about which words might cause difficulty.
Additionally, you could use Children's Printed Word Database (CPWD, 2013) which has word frequency data derived from books for 5 to 9-yo children. The words in this database should be safe to use for 13-yo readers.
For the word pocket square you could check the SUBTLEX-UK-bigrams. If the reader does not possess L1 comprehension, but learns English as a second language, the concept pocket square could be difficult. They would either need use a dictionary, or you could give the meaning yourself. From Wiktionary:
A handkerchief used as an accessory to a suit.
You could use a picture to illustrate what the concept means.
Search and Destroy
Pocket square might be a difficult concept for L2 English learners. Give a description of what it is:
A pocket square is a folded handkerchief in the breast pocket of a suit, used as an accessory.
The word standout can be found in the SUBTLEX-UK and has a Cbeebies (preschool) Zipf-value of 2.23 and a CBBC (primary school) Zipf-value of 2.47. They are both scores below 3, so better remove this word or replace it with a synonym. The word exceptional has better Zipf-values of 2.71 and 3.72.
The word vital has Zipf-values of 2.71 and 4.48. The word apparatus has Zipf-values of 2.53 and 3.43. Some readers might have difficulty with these words. You could rephrase by saying 'in order to become' or 'to be'.
A pocket square is a folded handkerchief in the breast pocket of a suit. It is an exceptionally vital accessory to be a stylish man. It's important to wear the correct pocket square for each event.
To summarize, in order to rephrase sentences so that 13-yo readers will understand them you could explain difficult concepts. And you could replace possibly difficult to understand words by more frequent used words that can be classified by children from age 5 to 12.