I didn't like this group so much when they were younger and more popular, but I started to like them and think they were cool when they got older
「大人になる」＝ become an adult / get older
「彼らが好き」＝ like them
「大人になってからの彼らが好き」＝ I started to like them when they got older / I started to like them when they become adults
「ボーカルグループで、若い頃の超人気でイケイケな時の彼らより、大人になり落ち着いた今の彼らがとても格好よくて好き。」＝ I didn't like this group so much when they were younger and more popular, but I started to like them and think they were cool when they got older
イギリス 
