25 is not late in your career these days, not even close. Almost all the top 15 are over 30, and there is hardly anyone in the top 100 over 20. 38 years olds (eg Serena and Federer) are winning slams. Mediocre players like Pennetta, Na, and Berdych are either winning slams or consistently staying in the top 10 into their 30s. Tennis is an older game than it has ever been, or atleast then it has been since the mid 60s. 25 is very early in your career for the standards of todays tennis.
I do not know how much Thiem will improve, largely as I do not even know if the is that talented or good to begin with, but he can potentially improve his level of play (especialy on clay) a ton.
Regardless of the state of the current tennis field, 25 is by no means 'very early' in a player's career and there are not many players who play better tennis later in their careers than earlier. Some are playing great tennis around the age of 30, but almost all of them were playing an equivalent level of tennis when they were around 25. Let's look at some examples:
Federer - best year 2006, age 24/25
Nadal - best year 2010, age 23/24
Djokovic - best year 2015 (age 27/28), but second best year 2011, age 23/24
Murray - best year 2016 (age 28/29), but second best period (2/4 slams won + Olympics) from 2012 Olympics - 2013 Wimbledon, age 25/26
Del Potro - best year 2009, age 20/21
Berdych - best year arguably 2010, age 24/25
Tsonga - best year 2008, age 22/23
Cilic - best year 2014, age 25/26
Raonic - best year 2016, age 25/26
Nishikori - best year 2014, age 24/25
Dimitrov - best year 2017, age 25/26
There are others who played their best tennis much later:
Anderson - best year 2018, age 31/32
Isner - best year 2018, age 32/33
Ferrer - best year 2013. age 30/31
Wawrinka - best year 2014/2015, age 28-30
But this isn't the new normal. The vast majority of recent players have played better in their mid 20s or at least as good. Anderson and Isner have very serve based games, which tends not to decline as players get older in the same way. They've also significantly benefited from the decline of the Big 3. Wawrinka took off because of Magnus Norman, who came in 2013. These guys are the exception, not the rule.
To reiterate, many players nowadays are playing some of their best tennis in their late 20s and 30s. However, very few of them weren't already playing at the same level in their mid 20s. The idea that Thiem is going to be playing a lot better in his late 20s than he is now is unfounded IMO