Boro park girl: Actually to become a licensed mechanical engineer (plumber, electrician, HVAC) you need to work for a licensed individual for 7 years. The licensing tests are also extremely difficult. They ask many questions about subjects you will never come across in the line of the work. (ie. ohms, resistance)
Most people who you see advertised in the phonebook that say they are licensed, are actually working under someone else’s license.