Learning a language at an older age is anyway difficult and will only succeed with a lot of effort. As someone that's learned a foreign language at almost 30 in a classroom, I can say that Duolingo does some things well, mostly by forcing or motivating you to repeat to memorize things. I say I learned it in a classroom, but objectively I never would've made it if I was not in the country whose official language I was learning. So the classroom gives you some basics and then you need to practice a lot until it becomes something you don't need to think about. As they say, internalize it. This takes years, but also depends on the language, some are more difficult or have different learning curves. Duolingo is again good at the practice part.

Most people try to use logic and comparisons to their native language, but this is imho not a good way. Kids don't learn like that. It's argued that grownups lose this ability, but personally I sometimes say thing where I have no idea why I said that and if it's really correct. But then it turns out it is. It's somewhere in my head and it got there by listening to other people speak or reading. I know a lot of people that got nowhere by going to class because they had their group of friends from e.g. Spain, China or Korea around them. I met a person from the US who was disappointed with the little progress they made and said "I don't understand that and why it's like that". My only answer was really "because it just is". So I think Duolingo has a place, as a sort of a virtual native speaker you can interact with.