If i had toenail fungus before, can i still get it again even when i'm older? is there a high chance of me getting it? should i have the same sheets?

Yes: Like any infection, fungal infections can recur in the same person even after successful treatment. However,it is fairly rare for that to happen. Ways you can reduce your chances include sterilizing or throwing away the shoes you wore when you had the infection, and adequately cleaning socks and sheets. If you do these things, you shouldn't have much to worry about.

Microscopic plants.: From the phrasing of your question, it is unclear as to whether or not you actually had a nail fungal infection. At your age, a fungal infection is relatively rare. Having said that, fungal spores are everywhere. It is impossible to avoid coming in contact with the spores. Hygiene, age, certain systemic conditions and just dumb luck are the major determinates of acquiring a fungal nail infection.

Yes: This is quite common. Repeat infection occurs for the same reason it happened already. This has nothing to do with sterilizing sheets & footwear. It is mostly about genetics. Ventilate your feet. Barefoot at home. Never sox in bed. Be ready to use antifungal cream for fungus on skin. This may show as scaling skin across the bottom of the foot and the webbing next to the smallest toe.