We're not referring to the general concept of a younger generation. We're referring to a specific, definite generation. Without more context I don't know whether we're talking about the generation of these skilled kids, who will be more difficult to surprise as they age and grow more sophisticated, or we're talking about the generation of kids at this age, who seem to gain an ever-growing set of skills by the time they enter the generation.
Whichever case it may be, the reference seems specific enough to require the definite article. It's "this younger generation", not "any younger generation", in question here.
There is a test that I find useful. I consider "the" to be a weakened form of "this", and "a" a weakened form of "one". If I can replace the article with the word "this" or "that", then "the" is an appropriate choice. If I can replace it with "one" or "any", then "a" is appropriate. On this basis,
- to surprise any younger generation
doesn't work as well in this sentence as
- to surprise this younger generation