I am 52 and still play tennis every day on hard courts at my club - usually about 4 singles matches and a couple of doubles matches every week along with a drill session or coaching lesson. I have about 10 guys I play singles with regularly (each guy roughly once every two weeks) and a list of about 15 guys I organize doubles matches with. The age of the players in my group varies widely and we play together because our playing level is roughly about the same - most of us are USTA 4.5s and there are a few with a high 4.0 or low 5.0 rating. My club will refer all new members who are at the 4.5/5.0 level to contact me and so, I keep getting new players to add to my group. I’ll keep guys on my invitation list if they make fair line calls and behave with good sportsmanship on the court - age does not matter. There are some guys in their early twenties who are just out of high school or played for a junior college while there are a couple of doubles players in their seventies who are Gold Ball winners for their age group. I know a 77-year old guy at my club with a current 4.5 computer rating who is a US national doubles champion for the 75+ age group - he is so physically fit and spritely that he is willing to crouch and play I-formation when his partner is serving throughout a match. He is an inspiration for the rest of us.
In many cases, the younger guys in their twenties and early thirties are slightly more mobile and hit bigger serves, but otherwise they don’t have any systemic advantage over the 4.5 players in their forties and fifties. This is because the younger guys are usually ex-high school players while most of the older guys are ex-college players or guys who have been playing USTA league at the 4.5 level for decades. The young guys under 40 who are ex-college players are still 5.0+ and are too good to play with my group - they have their own clique at our club. All the older guys who play singles regularly in my group are physically fit and that’s why they still continue to enjoy competitive singles - so, I can’t assume that an opponent’s mobility or endurance in a singles match will be limited by their age as many are swimmers, surfers, soccer/hockey players etc. along with playing singles in USTA leagues and tournaments. We’ll usually play a full third set instead of a 10-point tiebreaker to decide matches that are tied after two sets.
In my personal case, I do find that I‘ve lost back flexibility and some speed over the years, but don’t find much difference in endurance. So, kick serves and explosive quickness at the net are affected a bit compared to 15-20 years ago, but I don’t think ability to cover the court or play long rallies has been affected yet. I’ve started taking lessons once a week since I turned 50 to improve my game further - I feel like I need to keep improving my technique/footwork to make up for any physical decline that happens over time. My coach is in his late twenties and played College/Futures tennis less than 7 years ago - the main benefit of playing with him weekly is hitting against the high quality of his shots and the movement drills he makes me do. He does help with minor technique tweaks as he has added more topspin to my shots with an increase of shot tolerance and better footwork - so, I play less defensively than in the past against better players which helps make up for any potential decline in court coverage. I feel that I notice what’s working or not working during a match quicker and make tactical adjustments much faster than when I was younger as ‘experience‘ does come with playing thousands of matches. Also, doubles skills have improved a lot as I started playing doubles seriously only in the last decade after I turned forty. I’ve played all my life with heavy, thin-beam mid/mid-plus racquets starting with a Dunlop Max200G and play now with Pure Strike Tours strung with a gut/poly hybrid.
The other benefit of being over fifty is that many of us are empty-nesters and can devote more time to tennis now that our kids are grown up and we don’t have to spend our evenings and weekends taking them to after-school activities. The guys with young kids play much less than the older guys who are close to retirement or are retired. So, I don’t necessarily agree with those who say you need an oversize or very stiff racquet, have to stop playing singles, have to change technique, need to have rest days, need to change playing style etc. just because your age is over 50 or 55 - it really depends on your current fitness and tennis level. But, the need for stretching/workouts, good nutrition and a regular recovery program after matches definitely increases with age. And you can always keep improving your technique and footwork with the help of a good coach and solid drilling partners. There is usually a kernel of truth when a group is stereotyped, but you have to keep in mind that there can be a lot of variance within that group of people - so, not all older players can be thrown into one bucket and generalized about.
