Is Fluid Intelligence Dropoff With Age an Established Fact? The picture below represents the idea that fluid intelligence declines with age. Is this commonly accepted as true and established, even if fitness/learning levels do not decline?
You are asking two very excellent questions.
Is the decline in fluid intelligence commonly accepted as true?
These days it seems generally accepted, that the trend is true. However, this hasn´t been the case a few yeas ago, e.g. see the introduction in Elsayed, Ismail, &amp; Young (1980) for a little impression. The reason behind this is the limited comparability between different studies

due to different designs (longitudinal vs. cross sectional), that come with their own difficulties (f.e. longitudinal = drop out bias/survivorship bias, repetition bias; cross sectional = confounding of cohort and treatment effects - see Hofer &amp; Sliwinski (2001) for more details about that).
due to different intelligence tests, concepts and facets (= &quot;how do we define fluid/ crystallized intelligence&quot;), that have been used (e.g. some solely defined crystillized intellgence as vocublary, see Wang &amp; Kaufmann,1993).
of course there are many other aspects, that are not solely limited to the study of intelligence like small sample sizes, unrepresentative samples or publication bias.

However, as I mentioned at the beginning: these days - after a huge discussion and a lot of studies - it seems fairly accepted, see Verhaeghen &amp; Salthouse (1997) with a meta-analysis or Baltes (1996) for a review-like paper - Kaufman, Salthouse, Scheiber, &amp; Chen (2016) could also be relevant. The main reason behind this seems to be a decline in processing speed and working memory.
On a large scale this seems to come from a general (biological) decline across the lifespan (see Baltes &amp; Smith, 2004), which brings me to your second question.
... even if fitness/learning levels do not decline?
There are a lot of studies, that  show, that physical activity and general fitness seems to have benificial effects on fluid intelligence. This seems to hold for every age (see Elsayed, Ismail, Young (1980) or Linde &amp; Alfermann (2014)(an older sample)). The reasons behind this has not yet been clarified -

one can assume beneficial biological reasons like hightend blood circulation (see. Singh-Manoux, Hillsdon, Brunner, &amp; Marmot, 2005)
or the reduction of distress, which might enhance cognitive functioning.

I would recommend you to read the introductions and discussions of the cited papers (if possible) for further information. However, (sadly) I don't know any studies explicitly investigating the development of fluid intelligence across the lifespan in regard of exercise/physical fitness (I just know, that exercise/fitness has a beneficial effect on fluid intelligence, but how exactly this has an impact on the decline is - for myself- unknown.)
However, I would say, that there will be a decline in older age, which can be buffered by remaining exercising/a good level fitness, but not really stopped. There should still be a decline - at least at a smaller scale. Or as to say in the words of my professor: &quot;it won´t get better&quot;.
Of course all assumptions count for the general/average case, not for a single person.

Mohamed Elsayed, A. H. Ismail, R. John Young; Intellectual
Differences of Adult Men Related to Age and Physical Fitness Before
and After an Exercise Program, Journal of Gerontology, Volume 35,
Issue 3, 1 May 1980, Pages 383–387

Hofer, S. M., &amp; Sliwinski, M. J. (2001). Understanding ageing.
Gerontology, 47(6), 341-352.

Wang, J. J., &amp; Kaufman, A. S. (1993). Changes in fluid and crystallized intelligence across the 20-to 90-year age range on the K-BIT. Journal of Psychoeducational Assessment, 11(1), 29-37.

Verhaeghen, P., &amp; Salthouse, T. A. (1997). Meta-analyses of age–cognition relations in adulthood: Estimates of linear and nonlinear age effects and structural models. Psychological bulletin, 122(3), 231.

Kaufman, A. S., Salthouse, T. A., Scheiber, C., &amp; Chen, H. (2016). Age Differences and Educational Attainment Across the Life Span on Three Generations of Wechsler Adult Scales. Journal of Psychoeducational Assessment, 34(5), 421-441.

Baltes, P. B. (1997). On the incomplete architecture of human ontogeny: Selection, optimization, and compensation as foundation of developmental theory. American psychologist, 52(4), 366.

Baltes, P. B., &amp; Smith, J. (2004). Lifespan psychology: From developmental contextualism to developmental biocultural co-constructivism. Research in human development, 1(3), 123-144.

Linde, K., &amp; Alfermann, D. (2014). Single versus combined cognitive and physical activity effects on fluid cognitive abilities of healthy older adults: a 4-month randomized controlled trial with follow-up. Journal of aging and physical activity, 22(3), 302-313.

Singh-Manoux, A., Hillsdon, M., Brunner, E., &amp; Marmot, M. (2005). Effects of physical activity on cognitive functioning in middle age: evidence from the Whitehall II prospective cohort study. American journal of public health, 95(12), 2252-2258.