From Longman Dictionary of Contemporary Englishtender agetender ageYOUNGthe time when you are young or do not have much experienceat the tender age of four/five/six etc Nicholas was sent to boarding school at the tender age of seven. → tender
Examples from the Corpus
at the tender age of four/five/six etc• Alongside me was Sam Ratcliffe who, at the tender age of sixteen, had already had quite a bit of publicity.• And it has just happened to Kate Moss at the tender age of 18. 