too old to be reading the childish Harry Potter series.
I was 23 or something like that when I read Harry Potter and I still think it's a very good read, it's not the top of the literature writing art - but it is so good to read, an easy relaxing jorney with adventures and such. I liked despite reading lots of more serious books don't simplify Harry, it's a good book for every age : )
For Lem I'd probably suggest reading "Opowieści o Pilocie Pirxie", one of the more youth aimed books by Lem..