Young people want changes, but old people want things to stay the same. почему перед прилагательными young и old не стоит артикль?

Потому что people - множественное число и перед ними артикль не ставится

Артикли a/an не могут в данном случае стоять, поскольку слово people во множественном числе, причем оно не обозначает определенную расу/национальность.

Артикль the не подходит, потому что в данном случае словосочетания young people/old people подразумевают всю возрастную группу в целом, а не каких-то конкретных людей.

Также, the people может означать всё население страны, не участвующее в управлении государством, но в этом предложении речь совершенно о другом.

I want to tell about my best friend. His name is Alexander. He’s my school-mate. Sasha is cheerful, inquisitive and kind. He is very responsive. He will help me in hour of need. I always can rely on him. Sasha goes to sports school. He is very tall that’s why he goes in for basketball. On weekends we play football or cycle. Sometimes we go fishing and fish crucians. Also he is fond of playing computer games and watching horror films. His favourite films are “Saw” and “Scream”. We often go to the cinema. Sasha is true friend. My father says that I’m lucky because I have so reliable and true friend. I value our friendship.
МОЖЕШЬ СДЕЛАТЬ ПО ОБРАЗЦУ. только придется с переводчиком поработать тебе.

I love summer holidays. I like it becauseduring my holidays I can relax and have fun. I can do everything I like.  I don’t have to get up too early.  I don’t do my homework. I spent my holiday inCrimea. I like the  Black sea. On 5thJuly we left our town and went to Alushta Alushta is one of the largest townsin Crimea. It’s situated on the seaside between high beautiful mountains. Itwas a great time. We swimmed, walked by seaside, and visited many sights. Wehad an excursion to the Valley of the Ghosts, which is situated near Alushta.It was incredible sight, because I’ve never seen anything like this!Also we’vevisited the Dzhur-Dzhur waterfall, the Marble Cave, and the Red Cave. Thosedays were amazing and unforgettable; I wished if I had more days to spend overthere. My holidays were exciting and full of joy. I think that Crimea is a realparadise on earth. I dream to visit it again