Since plain form words can attach to つもり, we can just attach the i-adjective 若い to it as is.
When our plain form word is a NOUN, we'll add a の after it. And for na-adjectives, we add a な after them. For example, later we'll see:
in this grammar rule. We attach the つもり with the plain form (noun,adjective,verb). Is this つもり acting as a noun, because when its attached with an adjective, we place a な in between them, and with noun の.