The euro is growing up. Next month, the European Central Bank, the single currency's guardian, will mark its tenth anniversary. Soon after, in January, the euro itself will turn ten.

For the euro's enthusiasts, this is a moment of both vindication and celebration. The still-youthful euro, once ridiculed as a “toilet currency”, is, in many senses, now thriving.

The painful political birth pangs of monetary union are a fading memory; the euro's faltering early steps as its value plunged during its first two years have given way to its present, muscular strength; and in the foreign reserves of the world, the euro now stands second only to the once unassailable dollar.

Most crucially of all for the euro's proponents, the economy of the 15-nation eurozone is so far proving remarkably resilient in the face of the global credit crunch.

Whereas once the US economy's mocking call to its transatlantic challenger was “catch us if you can”, in the first quarter America slowed to the brink of stagnation while the eurozone notched up a healthy 0.7 per cent expansion.

At the eurozone's heart, Germany raced ahead with a stellar growth spurt that saw its GDP jump 1.5 per cent.

The eurozone's leaders should savour this comforting moment of economic Schadenfreude while they still can.

Even as the euro notches up its tenth year, there are increasingly danger signals that its 11th will usher in a troubling period of growing pains for Europe's monetary union.

The first of these looming problems is that although, until now, the eurozone seems to have demonstrated a surprising degree of immunity to the credit-crunch sickness, a catalogue of symptoms is emerging to indicate that it will soon succumb to this scourge.

Last week, surveys showed that growth across its services sector is close to stalling, with manufacturing also on course for stagnation.

More critical than the impending weakening of the eurozone's recovery is that this downturn seems destined to aggravate the fundamental problems of economic divergence that have beset the bloc since its creation.

It was clear from the start, a decade ago, that the disparate group of 12 founding economies slammed together in the single currency area could hardly have been more disparate.

At the time of the euro's conception, there were widespread warnings that the participants were far too dissimilar to be effectively steered with the single interest rate that a single currency requires.

Ten years on, and the critics have been vindicated, in spades. Claims that monetary union would bring member states into alignment have proved misplaced; economic convergence remains elusive. Instead, the “one-size-fits-all” interest rate policy of the ECB has turned out to mean “one-size-fits-none” - or almost none, at least.

This regime has entrenched, rather than eroded, the eurozone's deep economic faultlines. For much of the past decade, fast-expanding peripheral economies such as Spain and Portugal have run much too hot, with inflationary booms; others, especially Germany, have spent most of this time as laggards, running far too cold, and at times enduring near-deflationary conditions. Few eurozone members have seen policy set “just right”.

The consequence is that perilous imbalances have built up that now threaten to spark severe economic strains, and to inflame already simmering political stresses, as the credit crunch hits some parts of the eurozone much more viciously than others and triggers a reversal of fortunes for its members.

In Spain, which entered the euro as a relatively poor cousin, a run of rapid growth during a protracted period of catch-up is about to hit the buffers.

While Spain has prospered during this long boom, inappropriately low euro interest rates - made still lower in real terms by relatively high national inflation - have fostered a house price bubble that is now bursting with brutal repercussions.

At the same time, as David Owen, of Dresdner Kleinwort, notes, Spanish companies have plunged deep into the red, borrowing heavily to cash in on the boom, and swelling Spain's current account deficit to startling proportions as they suck in foreign capital.

All of this threatens a punishing adjustment as these imbalances unwind. Yet the ECB's ability to respond looks set to be hampered by the radically contrasting fortunes of Germany.

Its years of sub-par growth since the single currency's inception catalysed a drastic shakeout that has made it leaner, meaner, and more competitive.

In contrast to their heavily indebted Spanish rivals, German companies, as Mr Owen notes, are in a robust financial state with an overall surplus that will leave them well placed to weather the gathering storm as the credit crunch makes capital tough to raise.

Once again, then, the ECB faces being pulled in diametrically opposite directions by conflicting economic trends in key eurozone economies. As it battles to reconcile these forces, it seems inevitable that its continued struggle will ignite yet further tensions over economic management among Europe's leaders.

Already, the ECB is facing persistent, aggressive sniping from Nicolas Sarkozy and Silvio Berlusconi, the populist leaders of France and Italy.

With Spain skidding towards recession, they could well soon be joined in a powerful “axis of discontent” by José Luis Rodríguez Zapatero, Spain's Prime Minister. Yet Angela Merkel, the German Chancellor, is likely to keep up her staunch defence of the Frankfurt-based central bank.

The stage, then, is set for renewed political and economic strife among the single currency's members. The euro has survived its troubled childhood better than many critics predicted, but the decade until its coming of age looks likely to be just as testing as the ten years before.

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