Extremely common. Mandatory English classes in every level of education. Every person between 12-70 speaks English, but you will find children under twelve to have a more rudimentary grasp and elderly people who never learned English, so their English might be more difficult to understand.
Virtually everyone between 18-60 speaks at least Dutch, English and German, and most also speak French and/or Spanish and/or Arabic. It's quite common for Dutch office staff to speak 4-5 languages; I speak six plus a smattering of dirty words in another five languages.
My 13-year-old daughter is in the second grade of Gymnasium; she speaks/learns Dutch, English, German, French, Greek and Latin and is doing an extracurricular language course in Czech.
Most signs are in English and in many Amsterdam shops and cafes you cannot even order in Dutch because the staff isn't native Dutch. This is also why many expats do not learn Dutch; everybody speaks English and when they hear your accent they will switch over to English right away.