She is 14 years old. Her best friend is also 14 years old. (same)
→ Her best friend is the same age as her.
She is 14 years old. Her best friend is also 14 years old. (same)
→ Her best friend is the same age as her.
So sánh độ tuổi ⇒ dùng “the same + age + as”.
“She is” đứng cuối câu để tránh lặp lại "14 years old".
Dịch: Bạn thân của cô ấy cùng tuổi với cô ấy.