Are there any long term effects for untreated mild asthma?

Not really: Untreated asthma will cause difficulty breathing and routine inhalers such as albuterol will help. However there is no long term consequence of not treating asthma other than higher risk of hospitalizations from exacerbations.

Possibly: There is a normal decrease in lung function with age. Untreated asthma can worsen the decline in function. Asthma can be sneaky, slowly getting worse over time. You make think it's mild, but you're doing less and less without realizing it. I would recommend routine follow up and lung function testing.