I don't think pushing up the retirement age matters too much for youth unemployement, as those two groups compete for very different jobs (no 60+ year old will start vocational training as a nurse).
"How do we make the economy good" is obviously an extremely hard problem. I think a very important question is whether the 17% are actively seeking for jobs or considering themselves already entirely removed from the system.
France has a large industrial base, with many, many large companies (and suppliers for those companies), training people to have skills relevant to those companies seems an important consideration.
If that is the case you don't actually need more workers, but you have a very serious issue with young people leaving society and either by choice or circumstance not entering vocational training.
You do not need a 5 year degree to become a carpenter or nurse, if your country has a lack of those AND a significant unemployed youth population, the solution is obvious. Importing workers for those roles, just means that you are removing those training opportunities from your own population.
This has been a significant problem in the UK since brexit for farmers⁰: local workers are not willing to do that sort of work for that little pay¹ so the reduced availability of migrant workers has meant some crops simply went un-picked because the farmers were unable to afford to offer more³.
In other parts of the employment market where significant learning/experience is required before starting, there potentially are a different set of difficulties meaning jobs go unfilled despite there being unemployed people otherwise available.
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[0] and others offering seasonal or otherwise temporary physical work
[1] and/or in many cases unable to do it as fast² increasing costs to the employer
[2] though that problem would fix itself in time with practise, if people at least started the job
[3] as they'd have to swallow the cost on already miniscule margins in many cases, to the point where wasting part of a crop is financially better than paying more so they don't have to
Youth unemployment in France is at 17%.