First of all if they "were" born in the same year which year do you propose they be born in?
That's crucial, because timing is important.
It's one thing to argue that a 16-18 year Graf was somehow "better" than Navratilova, but time warp Steffi back to being born in 1956 and in 1974 she's 18 with a wood racquet playing the likes of Billie Jean King, Chris Evert, Evonne Goolagong AND Navratilova.
Guess what-THAT Steffi isn't winning THE Grand Slam ala 1988 style. I'm not even sure she's the best player under those conditions. That Graf might develop into the best later, but I doubt she's #1 out of the gate.
I am not talking about whether or not she wins as many slams as a teenager though. I am talking about her hypothetical head to head with Martina, which many have used such bold and frankly laughable claims as Martina would be doing far better than 9-9 born around the same time as Graf. And comparing the caliber of players both were at that point, even had they had the same equipment and faced the same fields and other variables, it is pretty clear Graf would have been owning Martina at that stage.
And Chris is only a couple years older than Martina. The majority of the slams back then were played on grass back then, where a prime Steffi is clearly superior to Chris. Of course this is an opinion, but I think you would agree nearly all the games experts would agree on a prime Graf being far superior to Chris on grass if asked. And Chris was number 1 already at that point. So I dont see it is far fetched Steffi could have been the number 1 player at that point. Probably not winning the Grand Slam, heck most years players werent even playing all 4 slams back then making it even less likely, but definitely a big presence on top. King was already aging and playing on beat up knees by then, and Evonne never had the mental toughness or consistency to dominate or be number 1.
The other alternative is to bring Martina forward. That would essentailly make her "free" (no need to defect) AND traveling with a full-time coach AND with topspin groundstrokes.
Fair enough but I dont think growing up in the graphite racquet and groundstroke heavy era would neccessarily favor Martina who would never have had the groundstrokes of people like Graf or Seles. She even admitted in the 90s to having frusteration with having to face so much power off the return and off the ground when trying to come in, from many more players than before, a byproduct of the racquet and the way the game was coached. If she is younger she might have tried to develop a game where she employed more topspin and stayed back more, but I am not sure that would have been in her favor neccessarily. Obviously not having to defect would have helped her emotional state though in her younger years.
As I wrote earlier, timing is important. Players peak at different ages. Players have opportunities handed to them at different stages of their careers. In 1975 Martina had to contend with a top ten field that consisted of 6 grand slam winners (Evert, King, Court, Wade, Richey, Durr). In 1988 a young Graf had to contend with an aging Navratilova (already 32) and Evert. The only other grand slam winner was a soon to retire Mandlikova.
Navratilova was actually 31 for Grafs Golden Slam in 1988, she did not turn 32 until all the grand slams that year were complete. King was the exact same age in 1975, she also turned 32 in late 75. So if Graf faced an aging Navratilova you feel, then Martina and number 1 ranked Evert likewise faced an aging King. Yet despite this fact Navratilova did not overtake King as a player for the first time until 1978, the year King turned 35 and Martina was 21.
In 1975 Court was even older, she turned 33 midway through that year. Durr whom you mention was 32 this year. Nancy Richey turned 33 in 1975 as well. These players were almost all just as aged as Navratilova and Evert in 1988 were in fact. Apart from Evert the youngest is Wade who turned 30 that year.
And since you mention all active slam winners, well in 1988 Mandlikova was another active slam winner at the time. She wasnt at her best by then, but the majority of players you listed werent in 1975 either making the point fairly moot. How many future slam winners were active in 1975 as well. I think that is also worth noting. In 1988 future slam winners who were active included Sabatini, Sanchez Vicario, Novotna, Martinez, most of whom were very close to Steffis age.
And another thing when it comes to a so called aging Martina. If people want to harp about what an incredibly late bloomer Martina was, how there are all these natural reasons it took her so long to develop, one cant then talk about her as a 30-32 year old in the same light you talk about a regular 30-32 year old. If she was such a late bloomer as everyone insists she was not far past her prime when Steffi began her dominance, unless one is arguing her longevity playing at her top level is lacking. I would also like to point out Martina in 1989-early 1990 won 67 of 70 matches at one point. Her only 3 losses were to Steffi Graf. If Graf did not exist she goes on a 70 match winning streak yet is supposably so far past her prime. Perhaps it is Graf who made her past her prime. Yet some people would have you believe Graf was lucky to face this Martina rather than the one who even as late as age 24 and 25 was still regularly losing matches to Pam Shriver, Andrea Jaeger, Wendy Turnbull, and Sylvia Hanika, often in slams and other major matches to boot.
Thus Graf had her golden chance handed to her at an earlier age. Martina had her golden chance at a later age.
It's all in the timing.
Graf may well have come in at a better time. However Navratilova and her early showings indicate quite clearly she wasnt anywhere near being a dominant player at the early age Graf was against any field. I understand the reasons Martina wasnt better at a younger age to a point, and the drawbacks she had. However I am just taking in light of those who seem to think Martina would clearly have a winning record against Graf if they were true contemporaries. I frankly dont see that at all.