Hi there !
Surprisingly, there is no more age verification plugin out therre (hum yes there is one on codecanyon, but not wordpress rule friendly (not gpl).
Could you do that ?
– crawler bots friendly (they will bypass verification)
– redirect link if not validated (disney by default ^^)
– custom BG or better integrated into a wp page (the homepage) directly for SEO purposes ?
Raccoon ? ^^