LONDON Alex Ferguson's retirement age, like Manchester United's goals against column, keeps rising and the link between them is becoming clearer as the manager strives for one last indelible triumph.
The Scot turned 71 on Monday, his competitive spirit apparently undimmed in his 27th year at Old Trafford and with his team seven points ahead of Manchester City in the Premier League title race.
Whether Ferguson will get to celebrate a 13th championship in May - not to mention the third European Cup victory he covets so much - is likely to rest on United's defensive strength.
Which is to say, their weakness.
Among myriad statistics this season, one has stood out; that in leaking 28 goals at the halfway mark, United have already conceded more than they did in the title-winning years of 2007 (27 goals), 2008 (22) and 2009 (24).
Strangely, Ferguson said this weekend when United recorded only their fourth clean sheet in the league with a 2-0 win over West Bromwich Albion that he had never had a stronger group of players at Old Trafford.
Yet their defensive frailty has been so glaring that even Ferguson, that notorious protector of players, has confronted it publicly.
Poor marking at set-pieces, lack of organisation, indecisive goalkeeping and the aging limbs of defenders Rio Ferdinand and Patrice Evra, and midfielders Paul Scholes and Ryan Giggs have all been blamed, but Ferguson is perplexed.
"I can't possibly answer it," he said after United came from behind three times to beat Newcastle 4-3 last week. "We've analysed it to every possible detail to try to get a constant thread that tells me how to address it.
"It's either don't attack at all, which is not going to be Manchester United, or let our supporters live through the agony of 4-3 wins. We've not had a 5-4 yet or a 6-5. I think that could be more exciting."
He was certainly joking about that, though his mood changes when you remind him how United lost the title to City last season - on goal difference on the final, stupendous afternoon of the campaign.
"It does matter, of course it matters," he said of the goals against column. "If we're scoring four goals at home there should be one against us or nothing. That's one way to help your goal difference. So that's always a concern.
"The concern about losing goals is that it's testing us to the very limit and making us play beyond the energy levels we need to."
Ferguson has been unable, or unwilling, to choose a No. 1 goalkeeper between David de Gea and Anders Lindegaard. In mitigation, the absence through injury of Rio Ferdinand, Phil Jones, Chris Smalling and, in particular, Nemanja Vidic has prevented United fielding a settled back four all season.
Vidic returned from three months out against Sunderland in December, and his continued presence beginning with the trip to Wigan Athletic on New Year's Day will be key to fulfilling United's ambitions in the next four and a half months.
"It is important we don't sit at the top of the table admiring our position," Ferguson said. "We would soon get a rude awakening. Congratulating ourselves on our Premier League lead doesn't mean a thing to me, it's winning the next game that counts."
Ferguson once said, when he was talking about the late Bobby Robson, that he would not be managing a Premier League club at the age of 70.
The fact he is still there at 71 - still remonstrating with referees and raging at his own players, including Nani, De Gea and Alex Buttner this season, is testament to his passion for the game.
Only recently he gave a lecture to the Harvard Business School in the United States, an unlikely venue for a fascinating insight into the methods of English football's most successful figure.
He covered several topics, from player power and managing millionaires to United's policy in the transfer market and how the Italian tenor Andrea Bocelli inspired a team talk.
"I had never been to a classical concert in my life," Ferguson said. "But I am watching this and thinking about the co-ordination and the teamwork - one starts and one stops, just fantastic. So I spoke to my players about the orchestra - how they are a perfect team."
There is, however, a more cynical way to view Ferguson's longevity.
That would be to conclude that the current United, trying to cope with the fading careers of Giggs and Scholes, shorn of giants such as Roy Keane and Cristiano Ronaldo and impatient for the development of young players like Danny Welbeck and Tom Cleverley, is not yet good enough to allow him to retire.
Steve Clarke, the West Brom manager beaten by his Scottish compatriot on Saturday, summed it up well.
"I have met a few grumpy 70-year-olds, and he is a typical grumpy Scot," Clarke said. "He likes to go chasing."
(Editing by Ed Osmond)