Can I take Lexapro (escitalopram) 20mg for years and years?

Yes. With any medication, you have to weigh the risks and benefits of that medication. If you are doing well on Lexapro (escitalopram) and develop no problems while taking it, keep on, keeping on.
Yes. As long as the Lexapro (escitalopram) is effective in controlling your symptoms and you are not experiencing any adverse effects it is certainly safe to continue on the Lexapro (escitalopram) at the most effective dose with periodic monitoring by your physician.