十几 = shí jǐ = dozen
(When 几 comes after 十 , it shows a quantity which is greater than 10 and less than 20)
十几个人 = shí jǐ gè rén = dozen people or so.
几十 = jǐ shí = dozens
(When 几 comes before 十 , it shows a quantity which is greater than 10 and less than 100)
几十个人 = jǐ shí gè rén = dozens of people.
十几岁 = shí jǐ suì = teenage
Wǒ shí jǐ suì.
I am teenager.