By Alan Baldwin

MONZA, Italy (Reuters) - Lewis Hamilton, once the young sensation of Formula One, is starting to feel his age as a new generation of drivers sweeps into the sport and old hands like Jenson Button and Felipe Massa head for the exit.

"For me, it is like 'Shoot - he was a part of my beginning and now he is stopping'," Mercedes' triple world champion told reporters after Brazilian Massa, 35, announced this season would be his last and he would bow out after 250 races.

"All these younger drivers are coming through -- there are a few ahead of me but bit by bit they will tail off and I will eventually be the oldest driver here, and then it will be my turn," added the 31-year-old Briton, who made his debut with McLaren in 2007 and beat then-Ferrari driver Massa to the title in 2008.

Former team mate Jenson Button's announcement at the Italian Grand Prix that he would be handing over his race seat at McLaren in 2017 to Belgian Stoffel Vandoorne will have only increased the feeling.

Button, the 2009 world champion with 301 grands prix to his credit, still has the possibility to drive in 2018, by which time he will be 38, but Formula is increasingly where the youngsters hang out.

Next year could also be the swansong of Ferrari's Kimi Raikkonen, the 2007 champion who is now 36, and possibly also McLaren's 35-year-old Fernando Alonso if that team does not improve markedly.

The men of the future are 18-year-old Max Verstappen, already in his second season and a race winner with Red Bull, Manor's Pascal Wehrlein (21) and Esteban Ocon (19), Vandoorne (24) and Toro Rosso's Carlos Sainz (22).

There is talk of Canadian Lance Stroll, who will be 18 in October and whose father is a billionaire, taking Massa's place at Williams.

"I think it is really exciting times," said Mercedes motorsport head Toto Wolff, who has Wehrlein and Ocon under contract.

"We were complaining a while ago that we had too many 'pay drivers' (those whose places depend on bringing sponsorship with them) in this sport. Right or wrong, these guys were actually not bad. Now we have a very exciting generation coming in.

"You have four drivers who have entered F1 purely on merit because of their records. I think this is great. Maybe we are just witnessing a generation change with the next generation of superstars," added the Austrian.

Of those coming in, or recently entered, Wehrlein became the youngest winner of the DTM German touring car series at the age of 20 in 2015 while Ocon won the GP3 championship in his rookie season.

Vandoorne enjoyed one of the most dominant championships in the GP2 support series with seven wins and 16 podiums in 2015 and has a bright future ahead of him.

"Stoffel’s contract is so long, it’s rolled up," joked McLaren head Ron Dennis. "His destiny is in his own hands and I’m pretty sure you’ll see Stoffel in the car for some time here."

(Editing by Amlan Chakraborty)