Will Finasteride save your hair and hairline for life?

To a varying extent, probably. It's definitely the best solution out there for preventing long term hair loss, but it isn't perfect. Some people see it begin to lose its effectiveness after several years, and for others it never stops the hair loss completely, just slows it down. There aren't really any other better options available, and it at least makes a significant difference for most men.

Hair loss is progressive throughout life.  I recommend patients start finasteride (among other medications) as soon as they notice hair loss as the earlier patients start the medications, the hope is that more hair is prevented from miniaturizing.  Having said that, hair loss is also unpredictable and each patient responds differently.