Beyond Mbappe, Rashford and Icardi, are we heading for a generation without elite center-forwards?

Where is the next generation of center-forwards going to come from?

You may notice it in the Champions League knockout rounds. Of the 16 teams left, how many have a proper, first-choice central striker under the age of 25?

I know definitions may vary, but bear with me. If you want to include Marcus Rashford and Kylian Mbappe, who are 21 and 20, respectively, and both played center-forward for their respective teams on Tuesday night, go ahead. But bear in mind that Mbappe is a freak of nature and generally plays wide for Paris Saint-Germain, while Rashford has also spent most of the past 18 months on the wing and it's by no means certain that his future is at center-forward.

Still, you can imagine both playing up front for a top team five years from now, and that makes them a rarity.

The fact is that most of Europe's top clubs either deploy center-forwards who are the wrong side of 30 -- Robert Lewandowski, Sergio Aguero, Mario Mandzukic, Luis Suarez, Edinson Cavani, Klaas-Jan Huntelaar, Karim Benzema, Edin Dzeko -- or they'll sometimes play guys up front who aren't traditional center-forwards. We've seen Liverpool do it with Mohamed Salah and Roberto Firmino, Atletico with Antoine Griezmann, Lyon with Memphis Depay, Borussia Dortmund with Marco Reus and Ajax on Wednesday night with Dusan Tadic.

In fact, apart from Mbappe and Rashford (who at least look the part) and Gabriel Jesus (who will have to contend with the Sergio Aguero roadblock for the next few seasons), the youngest Champions League center-forwards of any stature are actually in mid-career. There's a trio of 25-year-olds.

Nobody is going to argue with Harry Kane, but the other two come with asterisks: Romelu Lukaku appears to have lost his starting job at Manchester United and Paco Alcacer comes with an asterisk given the minimal impact he had in two seasons at Barcelona and the fact that, despite his prolific scoring this season, he's in and out of the starting XI at Dortmund.

You can debate whether Lukaku will regain his mojo or whether Alcacer will keep his, but the point is that if you're looking for quality under 25, it's slim pickings beyond Mbappe, Jesus and Rashford, all of whom already play for the very top clubs.

Expand the search across Europe's top leagues. We're around the halfway point of the 2018-19 season; it's not unreasonable to expect to see top-drawer center-forwards in double figures. And, indeed, there are 26 central strikers who fit the bill. How many of them are under age 25? Just five.

You've got Aleksandar Mitrovic (24) whose team, Fulham, is in the relegation zone. Krzysztof Piatek (23) is in his first season at Serie A and while he may or may not be a one-season wonder, this is his first year in a top league and, worryingly, he's been capped just twice by his native Poland. His countryman Arkadisuz Milik (24) has started 14 of 23 league game this season, and just seven over the previous two. Youssuf Poulsen (24) scored nine goals in 59 league appearances in the previous two campaigns. Sebastien Haller (24) has yet to be capped at the international level; then again, he's French, which might explain it.

Any of those guys excite you? In fact, does anybody besides Rashford, Kane, Mbappe, Jesus and all the thirtysomethings mentioned above excite you? Didn't think so.

I don't want to sound too negative about the potential drop-off. There are a few young center-forwards -- 21-year-old Yousself El Neysri at Leganes, 22-year-old Maxi Gomez at Celta, 19-year-old Rafael Leao at Lille -- who are generating a buzz, but it's slim pickings. And Slim is about to leave town.

We're heading toward a cliff-edge here, at least when it comes to Europe. Maybe South America or some other part of the world will step up and fill the void. If not, we're looking at a future where teams either reinvent players as center-forwards, play with a "false nine" or simply have front men who are functional rather than superstars. A bit like France, who won a World Cup with Olivier Giroud (and one with his precursor, Stephan Guivar'ch).

I've asked around about this trend and, anecdotally, have heard all sorts of theories to explain this.

Center-forwards mature later. (Really? Most of the veterans who inhabit the top sides were hugely prolific at a young age.)

Academies emphasise individual technique rather than scoring goals. (I don't see how skill stands in the way of scoring goals.)

It's just statistical variance! Deal with it, there will be another crop along in a couple years. (OK... but that feels like a cop-out.)

Whatever the case, the effects are already evident. Inter's long-running contract negotiation with Mauro Icardi (who reached 100 Serie A goals by age 23 and is now 25) underscores just what a commodity a reliable center-forward with another seven to eight years at the top is. Inter have put up with a lot (primarily with his wife/agent Wanda Nara) but it's somehow worth it because in a free market, rare commodities are extremely valuable.

Chelsea are paying nearly $30 million (in wages and loan fees) for the privilege of renting Gonzalo Higuain for six months, precisely because they needed a viable proven center-forward and there was nobody else around, and over the next few years, Bayern (Lewandowski), Barcelona (Suarez), Real Madrid (Benzema), Juventus (Mandzukic) and Chelsea (even if Higuain works out) will all be dealing with this issue.

That's a lot of very wealthy clubs that will be chasing the same tiny pool of talent.

If players were commodities -- and let's face it, to many that's what they are -- you'd probably be buying futures on center-forwards right now. Because it doesn't look like there will be that many A-listers for a while once this generation moves on. Economics 101 tells us that when the supply is limited, as long as the demand doesn't fall, the price goes up.