Are these sentences ok?
-It would take me about 15 mins to get to the university if I had a car by (the age of) 18. It's much less than taking the subway.
If I had a car by age eighteen, it would only take me about fifteen minutes to get to the university. That would be much faster than taking the subway.
Your sentences were grammatically correct, but the usage is somewhat awkward. The important thing, though, is that I was able to understand exactly what you meant. Keep trying, you're doing very well.
Thank you very much! That's great that my sentences were gramatically correct. I think that the reason why they sound a bit akward is that I'm not a native speaker and I don't have much practice I'll keep on trying - in Canada