March 12 (Reuters) - Mark Webber will be the oldest driver on the Formula One starting grid for his home Australian Grand Prix this weekend but suggestions he is reaching the end of the road sound premature to Red Bull principal Christian Horner.
"The last three years have apparently been Mark's last year, and yet he has been retained by the team because of what he is doing in the car," he told British reporters ahead of Sunday's Melbourne season-opener.
"There are an awful lot of drivers who would like to be sat in a Red Bull car, but he's there on merit, and whilst he delivers for the team he will have that place," added Horner.
Webber will be 37 this August and, out of contract with Red Bull at the end of the year after agreeing the latest in a series of one-year extensions, faces inevitable speculation about his future.
The Australian, who has never won his home race, came close to taking the championship in 2010 but lost out to German team mate Sebastian Vettel who is now, still only 25 years old, the youngest triple champion.
Red Bull advisor Helmut Marko caused a stir earlier in the year when he questioned Webber's mental strength and suggested he was a driver who could be unbeatable a couple of times a year but who struggled to maintain a title challenge.
"The difficulty for Mark is that he is constantly judged against a young man in the other car who has achieved so much," said Horner.
"What he (Vettel) has done in just over 100 races, with 25 victories, three-time world champion, youngest points scorer, pole winner, race winner, champion, is remarkable.
"He's being constantly measured against a driver that is, in my view, the best of a generation, and that makes it harder for Mark."
The Red Bull seat, with a team that has won the constructors' title for the past three years, is one of the most coveted in the sport.
There are plenty of candidates eyeing it up, including Australian Daniel Ricciardo and Frenchman Jean-Eric Vergne who drive for the Red Bull-owned Toro Rosso team and might be expected to graduate to the senior outfit.
Horner said Red Bull wanted the strongest driver pairing they could get.
"We're very happy with Mark going into what will be his seventh year with the team, the fifth consecutive year he will have been paired with Sebastian," he said.
"Mark has demonstrated that on his day he can be unbeatable, and he does just need to string a campaign together...he's improved as he's matured. Last year he drove very well.
"If we didn't think he could still do it then we wouldn't have taken him for another year with the team." (Reporting by Alan Baldwin, editing by Justin Palmer)