Johnny Galecki, who portrayed the sensitive physicist Leonard Hofstadter, was just 32 when the show began. Now at 49, Galecki reflects on Leonard’s journey through complex relationships, particularly with his neighbor Penny, played by Kaley Cuoco. Cuoco, now 39, captured hearts with her portrayal of the aspiring actress who initially seemed out of place among her scientifically-minded friends but grew into a strong, mature character throughout the series.