Assuming slam performances stay identical with respect to age for the sake of comparison, of course.
Which slams does Djokovic win in this hypothetical?
2007 Djokovic in 2001:
AO, RG, WB - obviously no
USO - no (loses to pre-final Sampras or Hewitt)
2008 Djokovic in 2002:
AO - obviously yes
RG - probably yes (beats Costa, probably beats pre-final Ferrero)
WB - obviously no
USO - no (loses to Sampras, probably Agassi and Hewitt too)
2009 Djokovic in 2003:
AO, RG, WB - obviously no
USO - probably no (I think he loses to Roddick but let's count it as a possibility to be generous)
2010 Djokovic in 2004:
AO - no (given he wilted vs Tsonga, Hewitt/Nalbandian/pre-final Safin/Agassi/Roddick should be too much)
RG - no (loses to pre-final Coria/Gaudio)
WB - no (loses to Roddick/Hewitt)
USO - probably no (I think he loses to Agassi but let's count this as a possibility)
2011 Djokovic in 2005:
AO - yes (beats Safin in epic 5)
RG - no (Nadal is already there and too strong)
WB - yes
USO - yes
2012 Djokovic in 2006:
AO - yes
RG - no
WB - yes (should beat Greendal in a competitive match)
USO - yes (no wind in this scenario and pre-wind Djokovic was peak)
2013 Djokovic in 2007:
AO - yes
RG - no
WB - no
USO - I think yes (struggles against Roddick and his younger self but beats them like he beat Wawrinka)
2014 Djokovic in 2008:
AO - no (loses to his younger self)
RG - no
WB - no (if Aulderer takes him to five there's no beating peakdal)
USO - maybe (given 2014 Noel lost to Nishikori, a loss to pre-final Murray or Djoko's younger self is on the cards)
2015 Djokovic in 2009:
AO - probably no (peakdal too good for strugglevic I reckon)
RG - probably yes (should beat del Potro in five aided by matchup)
WB - yes (roddick no slouch but peakovic too good)
USO - yes (beats Delpo again)
2016 Djokovic in 2010:
AO - yes
RG - no
WB, USO - obviously no (can't compete with peakdal in that form)
2017 Djokovic in 2011:
no slams obviously
2018 Djokovic in 2012:
AO, RG - obviously no
WB - maybe (50/50 vs Murray perhaps)
USO - maybe (loses to his 2012 self pre-final but if he's in the other half he could beat Murray and his younger self in the wind perhaps)
2019 Djokovic in 2013:
AO - maybe? (comes to 2013 vs 2019 Djokovic, far tougher draw in 2013 but the way he thumped Nadal in 19 was quite impressive so I don't know)
RG - no
WB - no (given the messy 2019 final he's not beating Murray there)
USO - no
2020 Djokovic in 2014:
AO - no (loses to his 2014 self, Wawrinka or pre-final Nadal)
RG - no
WB - doesn't exist in 2020 but likely no anyway
USO - not if he gets DQ'd
2021 Djokovic in 2015:
AO - no (loses to his 2015 self)
RG - no (loses to his 2015 self or Wawrinka most likely)
WB - no (loses to his 2015 self)
USO - no (loses to his 2015 self)
2022 Djokovic in 2016:
AO - would play without the vaccine debacle but loses to his 2016 self anyway
RG - no (loses to his 2016 self, possibly pre-final Murray/Wawrinka also)
WB - no (loses to Murray)
USO - would play without the vaccine debacle and may or may not win against his 2016 self and Wawrinka
2023 Djokovic in 2017:
AO - yes (beats Wawrinka/Nadal in a competitive match as Fed did)
RG - no
WB - yes (2017 field was quite weak, think none barring Fed - who is replaced here - were even up to Karl's level)
USO - probably no (I think he loses to 2017 Nadal but let's consider this a possibility)
Result:
6-8 AO
0-2 RG
4-5 WB
4-10 USO (what a wild range, speaks to Djoko's consistency but lack of sustained peak)
total: 14-25
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Now which slams does Federer win in this hypothetical?
2003 Federer in 2009:
AO, RG - obviously no
WB - yes (beats his 09 self and Roddick)
USO - obviously no
2004 Federer in 2010:
AO - probably yes (I think he beats his 2010 self)
RG - no
WB - yes (beats Nadal in a tough match)
USO - probably yes (I think he beats Nadal but let's consider this a possible loss)
2005 Federer in 2011:
AO - yes (beats his 2011 self and Murray easy)
RG - no
WB - yes
USO - yes
2006 Federer in 2012:
AO - probably no (loses to Nadal in that up-and-down form I suppose, but a win is possible)
RG - no
WB - yes
USO - yes
2007 Federer in 2013:
AO - yes
RG - no
WB - yes
USO - probably yes (suppose a loss to Nadal is also possible)
2008 Federer in 2014:
AO - maybe (Wawrinka and pre-final Nadal tough obstacles)
RG - no
WB - yes (beats his 2014 self)
USO - yes (beats Cilic and co)
2009 Federer in 2015:
AO - yes (beats Wawrinka and Murray)
RG - probably yes (should beat Wawrinka)
WB - yes (beats his 2015 self)
USO - yes (beats his 2015 self)
2010 Federer in 2016:
AO - yes (beats his 2016 self and Murray)
RG - maybe (may lose to pre-final Murray/Wawrinka but beats final Murray and co, I think)
WB - no (loses to Murray)
USO - maybe (I think he beats 2016 Djokovic and Wawrinka, but not a given)
2011 Federer in 2017:
AO - yes (beats his 2017 self and Nadal, there I said it)
RG - no
WB - yes (no peak tsonga to upset him in the 2017 field)
USO - yes (beats 2017 Nadal)
2012 Federer in 2018:
AO - yes (beats his 2018 self)
RG - no
WB - yes (beats 2018 Nadal)
USO - maybe (may or may not beat 2018 del Potro)
2013 Federer in 2019:
AO - maybe (even that Federer could probably wear down 2019 AO Nadal)
RG, WB, USO - obviously no
2014 Federer in 2020:
AO - probably yes (I think he beats Thiem and co, but not an autowin probably)
RG - no
WB - N/A (sad)
USO - probably yes (may lose to Thiem pre-final but beats him in the final and beats the rest)
2015 Federer in 2021:
AO - obviously no
RG - no (though I imagine the Nadal match would be actually competitive)
WB - yes
USO - yes
2016 Federer in 2022:
AO - yes (Med/Nadal wouldn't be getting a set off peakovic I bet)
RG - absent
WB - maybe (the 2022 field is so poor even Hobblerer could beat it unless he collapses, which is possible I suppose)
USO - absent
2017 Federer in 2023:
AO - yes
RG - absent (though I imagine he may play knowing that Nadal is absent, but let's keep the comparison clean)
WB - yes
USO - maybe (don't trust Medvedev/Alcaraz here)
TBD:
2018 AO in 2024, 2019 RG/WB in 2025, other post-2017 showings are not slam-winning forms in any case
Result:
8-13 AO
0-2 RG
11-12 WB
6-12 USO
total: 25-39 + 0-3 TBD (massive range here, speaks to Federer's consistency and being tough to put away)
Nice to have the difference spelled out so clearly. I didn't engineer the results before I did the count, if you're wondering. Looks like Djokovic is trailing pretty strongly in this age-adjusted hypothetical. What it tells us, hmm?