When I first played in a band in the 1960s learning chords and lyrics - and lead guitar breaks in my case - was not a problem. It was only when I hit my late 50s that I found I needed song sheets, mainly to remind me of the arrangement, e.g. which verse I solo over, and lyric prompts: usually the opening line suffices. I'm 67 now and envy you guys that can still commit songs totally to memory, I can't, no matter how many times I play them.
I know I'm not alone in this; it's not advancing dementia or anything, just one of those age things. I've seen artists on TV such as Brian Ferry and Paul Heaton of Beautiful South both openly using song sheet books on music stands, even though they've written the songs themselves. Some prefer to be more discreet, for example Ian Gillan, lead singer of Deep Purple, who uses scrolling teleprompters placed near his floor monitors. I watched Sting playing live on TV the other night doing the same.
So I bind all my lead sheets in a booklet and refer to one as and when I need it and don't worry about it anymore.