I think it's a "Czech has only one present tense thing," as a result of which the same Czech verb form can have several different meanings when translated to English.

The course USUALLY seems to translate Czech present tense as present continuous in English, and sometimes as the simple present. But neither of those would work here because the "doing" has been going on "since I was thirteen" and is still going on (which we know because the verb is "dělám").

I hope that makes some sense, but maybe a native speaker will provide a more authoritative explanation.

In Dutch it's also just a present simple. "Ik doe dat sinds mijn dertiende." I had to wrap my mind around the translation until I realised it barely differs from my native tongue after all. ;)

We only use an ordinal whereas Czech say "my thirteen of years" (literally), the rest is the same as in Dutch. :p

In English the sentence would most often be constructed just the way it is shown at the top of this discussion. Another good translation would be "...since I was thirteen YEARS OLD." But "...since I was thirteen YEARS" would not be used by a native speaker.

Léto can mean rok as well. It is usually used in plural let when used in this sense. Good dictionaries should contain this sense

The SSČ dictionary:

léto, -a s (mn. 2. let, 3., 6., 7. le- i lé-)

1. roční období od 21. 6. do 22. 9.: trávit léto v přírodě

2. zprav. mn. rok 1: před dvěma lety; před léty zemřel; známe se už léta dlouho; za mladých let v mládí

1. léta letokruhy: léta na pařezu

♦ babí léto pavučinová vlákna poletující vzduchem koncem léta; toto období spojené s pěkným počasím; přen. začátek podzimu, září; být sto let za opicemi expr. zastaralý, zaostalý; v zimě v létě stále;