Hello Bummer Man,
As long as you have a legal lifetime license and the receipt for it then yes, it will always continue to always be valid. The issue is that many unscrupulous people have illegally sold lifetime licenses derived from cracks, keygens, etc. If one of those were sold to you from places like eBay or similar then it is quite possible the license is not valid. You can contact myself or our Helpdesk if you'd like us to check on your license for verification.
Thank you
Ron, Forum Community Manager at Malwarebytes