not 100% sure but yoshino probably 18/19 since she's about to enter college
Yoshino became 18 at the start of chapter 1 when she moves to Kirishima's school. The school year in Japan begins in early april and Yoshino was born on the 2nd of april and since this is her last year of highschool before she enters college, Yoshino is 18 at the start of the manga. By chapter 38 Yoshino is 18 and a half (taking place during autumn when the weather gets colder) and since Kirishima's birthday is in november, it will soon be his birthday. Let's hope the author will end the hiatus and we will see Kirishima's 18th birthday.
In chap 1, Renji said that Yoshino will be turning 17 so she’s still 16 years old. But in vol.2 chap 9.9, Shouma was introduced to be older than Yoshino by 2 years but bro was already 20 years old. So um, what the heck? How old is Yoshino??