late bloomer

伊坂幸太郎氏の小説 "Remote Control" を読んでいます。パート4，The Incidentから引用します。

"Well, then," Morita said, leaning forward, "how about that Rinka. Did you do it with her?" His tone was neutral but the intentness in his look make Aoyagi wince.
"Are you serious?"
"But you were her knight in shining armor, you came to her rescue. Didn't she owe you something? I'm sure you got friendly. So, did you do it?" This was the same old Morita, getting all worked up at the mention of a woman and wanting to know everything in detail. In reality, though, he had been something of a late bloomer himself; shy as anything when left alone with a girl, usually not even getting around to holding hands by the end of a date.

"late bloomer" を辞書で見ます。
・Wiktionary: A person who achieves proficiency or success in a field significantly later in life.: He was a late bloomer, finding success at 40 in Luc Besson's 1988 film The Big Blue.
・Farlex Dictionary of Idioms: Someone who develops their talents or capabilities later than is typical among their peers.: It took my brother until he was 35, but he finally found a stable career that he loves. I guess he was just a late bloomer.

"late bloomer" は米国英語で英国英語では "late developer" を使うとありました。
・Collins Dictionary: someone, esp a teenager, who matures physically or emotionally at an older age than considered usual: he was labelled as 'a late developer' at his school.