Typically, they are stated to be 15 years old. Leonardo has occasionally been given the age of 16 in some media outlets due to his seniority, but they are generally considered to be the same age when transformed from pet turtles.
They had multiple birthdays in the Mirage comics, indicating that they were all the same age (If I remember correctly, their 17th and 18th birthdays were shown, and much later were stated to be in their 30s).
In the IDW continuity, ages are not specified, but the Turtles are reincarnated from Hamato Yoshi’s actual sons in Feudal Japan, who appear to have age differences, implying that the Turtles in modern times would be the same physical age (around 3–5 years old), but would have varying levels of mental maturity.
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Leo is clearly the eldest here, followed by Donatello; Raphael was a toddler in his first appearance, and Mikey required assistance to escape following their mother’s death.
When the Foot arrives to assassinate Yoshi and his sons, they are shown to be quite elderly (screenshot below).
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Assuming they are between 9 months and 2 years apart (while birth control was available, the spiritual Shinobi may have been opposed to its use), and they are all actual adolescent boys, Mikey would have to be 13 (generally considered the start of adolescence) and Leo no older than 19 when they are killed in Feudal Japan by the Foot in human form.