diff --git a/src/engine.cpp b/src/engine.cpp
--- a/src/engine.cpp
+++ b/src/engine.cpp
@@ -675,7 +675,7 @@ void SetYearEngineAgingStops()
  * @param aging_date The date used for age calculations.
  * @param seed Random seed.
  */
-void StartupOneEngine(Engine *e, TimerGameCalendar::Date aging_date, uint32_t seed)
+void StartupOneEngine(Engine *e, const TimerGameCalendar::YearMonthDay &aging_ymd, uint32_t seed)
 {
 	const EngineInfo *ei = &e->info;
 
@@ -699,7 +699,11 @@ void StartupOneEngine(Engine *e, TimerGa
 	 * Note: TTDP uses fixed 1922 */
 	e->intro_date = ei->base_intro <= TimerGameCalendar::ConvertYMDToDate(_settings_game.game_creation.starting_year + 2, 0, 1) ? ei->base_intro : (TimerGameCalendar::Date)GB(r, 0, 9) + ei->base_intro;
 	if (e->intro_date <= TimerGameCalendar::date) {
-		e->age = (aging_date - e->intro_date).base() >> 5;
+		TimerGameCalendar::YearMonthDay intro_ymd = TimerGameCalendar::ConvertDateToYMD(e->intro_date);
+		int aging_months = aging_ymd.year.base() * 12 + aging_ymd.month;
+		int intro_months = intro_ymd.year.base() * 12 + intro_ymd.month;
+		if (intro_ymd.day > 1) intro_months++; // Engines are introduced at the first month start at/after intro date.
+		e->age = aging_months - intro_months;
 		e->company_avail = MAX_UVALUE(CompanyMask);
 		e->flags |= ENGINE_AVAILABLE;
 	}
@@ -756,10 +760,11 @@ void StartupEngines()
 {
 	/* Aging of vehicles stops, so account for that when starting late */
 	const TimerGameCalendar::Date aging_date = std::min(TimerGameCalendar::date, TimerGameCalendar::ConvertYMDToDate(_year_engine_aging_stops, 0, 1));
+	TimerGameCalendar::YearMonthDay aging_ymd = TimerGameCalendar::ConvertDateToYMD(aging_date);
 	uint32_t seed = Random();
 
 	for (Engine *e : Engine::Iterate()) {
-		StartupOneEngine(e, aging_date, seed);
+		StartupOneEngine(e, aging_ymd, seed);
 	}
 	for (Engine *e : Engine::Iterate()) {
 		CalcEngineReliability(e, false);
diff --git a/src/engine_func.h b/src/engine_func.h
--- a/src/engine_func.h
+++ b/src/engine_func.h
@@ -27,7 +27,7 @@ bool IsEngineBuildable(EngineID engine, 
 bool IsEngineRefittable(EngineID engine);
 void SetYearEngineAgingStops();
 void CalcEngineReliability(Engine *e, bool new_month);
-void StartupOneEngine(Engine *e, TimerGameCalendar::Date aging_date, uint32_t seed);
+void StartupOneEngine(Engine *e, const TimerGameCalendar::YearMonthDay &aging_ymd, uint32_t seed);
 
 uint GetTotalCapacityOfArticulatedParts(EngineID engine);
 
diff --git a/src/saveload/oldloader_sl.cpp b/src/saveload/oldloader_sl.cpp
--- a/src/saveload/oldloader_sl.cpp
+++ b/src/saveload/oldloader_sl.cpp
@@ -399,6 +399,7 @@ static bool FixTTOEngines()
 	}
 
 	TimerGameCalendar::Date aging_date = std::min(TimerGameCalendar::date + CalendarTime::DAYS_TILL_ORIGINAL_BASE_YEAR, TimerGameCalendar::ConvertYMDToDate(2050, 0, 1));
+	TimerGameCalendar::YearMonthDay aging_ymd = TimerGameCalendar::ConvertDateToYMD(aging_date);
 
 	for (EngineID i = 0; i < 256; i++) {
 		int oi = ttd_to_tto[i];
@@ -407,7 +408,7 @@ static bool FixTTOEngines()
 		if (oi == 255) {
 			/* Default engine is used */
 			TimerGameCalendar::date += CalendarTime::DAYS_TILL_ORIGINAL_BASE_YEAR;
-			StartupOneEngine(e, aging_date, 0);
+			StartupOneEngine(e, aging_ymd, 0);
 			CalcEngineReliability(e, false);
 			e->intro_date -= CalendarTime::DAYS_TILL_ORIGINAL_BASE_YEAR;
 			TimerGameCalendar::date -= CalendarTime::DAYS_TILL_ORIGINAL_BASE_YEAR;