How many years do you have? That is a common but very incorrect question in English. We don’t ‘have’ years, as in Czech. Instead, we ‘are’ years and you need to add the word ‘old’. So, instead of saying, „I have 40 years“, you have to say, „I am 40 years old.“
Please note, however, that when used as an adjective, we drop the ‘s’ and link the phrase together with hyphens. For example, we don’t say, „He is a 40 years old man“, we say: „He is a 40-year-old man.“
Another incorrect utterance is so say ‘since my X years’ when referring to a time from when something started. My students might say, for example, „I have known Jakub since my 4 years.“ But again, this is wrong in English.
Instead, you have to say, „I have known Jakub since I was 4 years old.“ Or in this case, it is actually fine to drop the ‘years old’ and just say, „I have known Jakub since I was 4.“