The German statistical office has published the age pyramid of Germany as of January 1, 2015, so we can compare the age structures of Germany and France for one more year (France published its age pyramid as of Jan. 1, 2015 last January).
The main news from these 2015 age pyramids is France now has more inhabitants than Germany at all ages from 0 to 16. Last year (Jan. 1, 2014), France had more inhabitants than Germany at all ages only from 0 to 15.
The huge inflows of immigrants in Germany is nonetheless being felt. Last year, France had as many people as Germany for age 17 and age 18 (oddly, Germany had more people than France at age 16, i.e. people born in 1997, whereas France, like I said, had more people than Germany at all ages from 0 to 15). Well this year, these people are now 18 and 19, and these two generations are now significantly more numerous in Germany than in France (whereas they were equal last year). That clearly shows that many migrants born in 1995 and 1996 arrived in Germany in 2014.
I've made a calculation for this migration effect. I've compared the difference between France and Germany for each generation last year and this year. The calculations show a clearly pattern of migrant bulge in Germany from age 18 to age 33.
For age 18 (i.e. people born in 1996), France and Germany had the same number of people from this generation last year, but this year (Jan. 1, 2015) Germany has 20,000 more people from this generation than France (806,000 in France vs 826,000 in Germany). So for the people born in 1996 who were 17 last year and who are 18 this year, Germany has increased its lead over France by 20,000. That's pretty big (2.5% of that generation have arrived in Germany last year alone, net of departures).
Like I said, Germany increased its lead over France in a very significant way from age 18 to age 33 (after age 33, Germany increased its lead over France only slightly, and after age 55 it's actually France which increased its lead over Germany, probably due to higher life expectancy in France + departures of German retirees to southern European shores; France also increased its lead over Germany from age 3 to age 9, which tends to show that immigrants arrive in Germany without children, whereas immigrants arrive in France with young children).
The two generations where Germany increased the most its lead over France are those from 1991 and 1992. For the people from 1992, who were 21 in Jan. 2014 and 22 in Jan. 2015, Germany increased its lead over France by a whopping 40,000! For the people from 1991, who were 22 in Jan. 2014 and 23 in Jan. 2015, Germany increased its lead over France by 37,000. So apparently 22-23 y/o is the age when migrants are most likely to move to Germany. France on the other hand... well the country does its best to repulse migrants.
So it looks like next year France will have more inhabitants than Germany at all ages from 0 to 17, but it remains to be seen whether in the next years France will be able to have more people than Germany at ages 18, 19, 20, etc. With current high migration to Germany, it's quite possible that Germany can keep its lead over France for those ages until 2020 (without immigration, Germany would lose its lead over France for age 18 as soon as Jan. 2017, for age 19 as soon as Jan. 2018, etc).
It seems impossible for Germany to keep its lead over France at those ages for ever though. The coming French generations (those which are less than 15 y/o now) are more and more numerous compared to the same German generations, so it would require impossibly high levels of immigration for Germany to still be more populous at those ages (18, 19, 20s) than France.
For example, the people born in 2000, who will be 18 in Jan. 2019, are 67,500 more numerous in France than in Germany. The people born in 2001, who will be 18 in Jan. 2020, are 83,000 more numerous in France than in Germany. The people born in 2004, who will be 18 in Jan. 2023, are 101,000 more numerous in France than in Germany. The people born in 2006, who will be 18 in Jan. 2025, are 160,000 more numerous in France than in Germany.
Given that at best in 2014 Germany increased its lead over a single French generation by 40,000 (and this with record immigration in the country), it seems almost certain that France, even with super low immigration levels, will pass Germany for the people 18-25 y/o in the early 2020s.
I've detailed all these figures, because they are not some forecasts or population projections. They are real observations of each country's population, and they help understand what's going to happen. Not much can change this scissor effect for the next 20 years, unless Germany suddenly had net migration of more than 1 or 2 million per year (which would mean 2 to 4 million new arrivals per year