Neither of those races has a particular great record in predicting GT winners, races like Romandie and Algarve probably have more future GT winners in their winners' list (I'm too lazy too cross-check).
Back in the day Avenir was not restricted by age iirc. You could have 28 year olds competing with 19 year olds. Not sure when exactly it changed, but looking at the winners since 2000, you have Mensjov, Quintana, Bernal, Pogacar. Then there is also Lopez, Chaves, Mollema, Gaudu. Expanding to top 10 finishers, you also get names like Hindley, Arensman, Almeida, Van Wilder, Lambrecht, Vlasov, Hart, Haig, Oomen, G Martin, De Plus, Latour, Buchmann, Yates twin, Konrad, Formolo, Cattaneo, Barguil, Bardet, Kelderman, Caruso...
But recently, the Norwegian winners took their final chance in the U23 before being too old to compete, in order to beat contenders up to 3 years younger than them. In that case (as big talents leave u23 sooner) i don't think it's that much of a reference when you win as a 22 year old against 18 and 19 year olds.