Dexter’s age isn’t frequently mentioned, either in the original series or in the first few episodes of “New Blood,” so it makes sense that fans would be curious about how old the serial killer is. Thankfully, the original series reveals Dexter’s birthday in Season 8 via the character’s driver’s license, giving us a place to start. The character is born on February 1, 1971 — which is also actor Michael C. Hall’s birthday (via Biography).
As for when the revival takes place, the Showtime synopsis confirms that “Dexter: New Blood” is set exactly 10 years after Dexter fakes his own death in the finale of the original series. The “Dexter” finale, “Remember the Monsters?”, aired on September 22, 2013, but the episode actually takes place in the fall of 2012, with September 6, 2012, being the date that Dexter’s destroyed boat is found (via official “Dexter” wiki). Knowing this, it appears that “New Blood” is set in the fall of 2022, which would make Dexter most likely 51 years old in “Dexter: New Blood.”
As for his son Harrison’s age, he is probably around 15 years old. Previously, Dexter mentions in Season 7 that Harrison is three. Season 8 then begins six months after the Season 7 finale, but it’s unclear how much time passes before the Season 8 finale. According to estimations, Harrison is likely about 15 or 16 in “New Blood.” In the new series, Angela’s daughter, Audrey (Johnny Sequoyah) — who is implied to be the same age as Harrison, but it’s never explicitly stated — is frequently seen driving, making her likely at least 16. So, there’s also the chance that Harrison has been aged up a year or so in the reboot, but nothing’s been confirmed.